tests/resource/aws_codebuild_project: Add test covering environment v…

…ariable removal

References:

* hashicorp/terraform#20505
* #6427

The `environment` configuration block `environment_variable` configuration block came up in our `Optional: true` and `Computed: true` discovery as potentially problematic in Terraform 0.12 when the ability to use attribute syntax to zero out the configuration will require special implementation. However, it appears the functionality will actually continue to work due to the `environment` configuration block `Set` function and how updates of it are implemented.

Here we still add the covering test for when the `environment` configuration block attribute is switched from `TypeSet` to `TypeList` during future simplification work. While it would probably be okay to just remove `Computed: true` now, we take the less risky approach of just leaving this as-is for now until that simplification work is completed.

Output from acceptance testing:

```
--- PASS: TestAccAWSCodeBuildProject_Environment_EnvironmentVariable (37.91s)
```

aws/resource_aws_codebuild_project_test.go

@@ -269,6 +269,56 @@ func TestAccAWSCodeBuildProject_EncryptionKey(t *testing.T) {
 	})
 }
 
+func TestAccAWSCodeBuildProject_Environment_EnvironmentVariable(t *testing.T) {
+	var project1, project2, project3 codebuild.Project
+	rName := acctest.RandomWithPrefix("tf-acc-test")
+	resourceName := "aws_codebuild_project.test"
+
+	resource.ParallelTest(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t); testAccPreCheckAWSCodeBuild(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckAWSCodeBuildProjectDestroy,
+		Steps: []resource.TestStep{
+			{
+				Config: testAccAWSCodeBuildProjectConfig_Environment_EnvironmentVariable_One(rName),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckAWSCodeBuildProjectExists(resourceName, &project1),
+					resource.TestCheckResourceAttr(resourceName, "environment.1380979031.environment_variable.#", "1"),
+				),
+			},
+			{
+				ResourceName:      resourceName,
+				ImportState:       true,
+				ImportStateVerify: true,
+			},
+			{
+				Config: testAccAWSCodeBuildProjectConfig_Environment_EnvironmentVariable_Two(rName),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckAWSCodeBuildProjectExists(resourceName, &project2),
+					resource.TestCheckResourceAttr(resourceName, "environment.4178155002.environment_variable.#", "2"),
+				),
+			},
+			{
+				ResourceName:      resourceName,
+				ImportState:       true,
+				ImportStateVerify: true,
+			},
+			{
+				Config: testAccAWSCodeBuildProjectConfig_Environment_EnvironmentVariable_Zero(rName),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckAWSCodeBuildProjectExists(resourceName, &project3),
+					resource.TestCheckResourceAttr(resourceName, "environment.2300252877.environment_variable.#", "0"),
+				),
+			},
+			{
+				ResourceName:      resourceName,
+				ImportState:       true,
+				ImportStateVerify: true,
+			},
+		},
+	})
+}
+
 func TestAccAWSCodeBuildProject_Environment_EnvironmentVariable_Type(t *testing.T) {
 	var project codebuild.Project
 	rName := acctest.RandomWithPrefix("tf-acc-test")
@@ -1155,6 +1205,93 @@ resource "aws_codebuild_project" "test" {
 `, rName)
 }
 
+func testAccAWSCodeBuildProjectConfig_Environment_EnvironmentVariable_One(rName string) string {
+	return testAccAWSCodeBuildProjectConfig_Base_ServiceRole(rName) + fmt.Sprintf(`
+resource "aws_codebuild_project" "test" {
+  name         = %[1]q
+  service_role = "${aws_iam_role.test.arn}"
+
+  artifacts {
+    type = "NO_ARTIFACTS"
+  }
+
+  environment {
+    compute_type = "BUILD_GENERAL1_SMALL"
+    image        = "2"
+    type         = "LINUX_CONTAINER"
+
+    environment_variable {
+      name  = "SOME_KEY"
+      value = "SOME_VALUE"
+    }
+  }
+
+  source {
+    type     = "GITHUB"
+    location = "https://github.com/hashicorp/packer.git"
+  }
+}
+`, rName)
+}
+
+func testAccAWSCodeBuildProjectConfig_Environment_EnvironmentVariable_Two(rName string) string {
+	return testAccAWSCodeBuildProjectConfig_Base_ServiceRole(rName) + fmt.Sprintf(`
+resource "aws_codebuild_project" "test" {
+  name         = %[1]q
+  service_role = "${aws_iam_role.test.arn}"
+
+  artifacts {
+    type = "NO_ARTIFACTS"
+  }
+
+  environment {
+    compute_type = "BUILD_GENERAL1_SMALL"
+    image        = "2"
+    type         = "LINUX_CONTAINER"
+
+    environment_variable {
+      name  = "SOME_KEY"
+      value = "SOME_VALUE"
+    }
+
+    environment_variable {
+      name  = "SOME_KEY2"
+      value = "SOME_VALUE2"
+    }
+  }
+
+  source {
+    type     = "GITHUB"
+    location = "https://github.com/hashicorp/packer.git"
+  }
+}
+`, rName)
+}
+
+func testAccAWSCodeBuildProjectConfig_Environment_EnvironmentVariable_Zero(rName string) string {
+	return testAccAWSCodeBuildProjectConfig_Base_ServiceRole(rName) + fmt.Sprintf(`
+resource "aws_codebuild_project" "test" {
+  name         = %[1]q
+  service_role = "${aws_iam_role.test.arn}"
+
+  artifacts {
+    type = "NO_ARTIFACTS"
+  }
+
+  environment {
+    compute_type = "BUILD_GENERAL1_SMALL"
+    image        = "2"
+    type         = "LINUX_CONTAINER"
+  }
+
+  source {
+    type     = "GITHUB"
+    location = "https://github.com/hashicorp/packer.git"
+  }
+}
+`, rName)
+}
+
 func testAccAWSCodeBuildProjectConfig_Environment_EnvironmentVariable_Type(rName, environmentVariableType string) string {
 	return testAccAWSCodeBuildProjectConfig_Base_ServiceRole(rName) + fmt.Sprintf(`
 resource "aws_codebuild_project" "test" {