Ensure only plugin REST tests are run for plugins (#53184)

This commit fixes ensures that for external builds
(e.g. plugin development) that the REST tests that are
copied are properly filtered to only include the API
by default.

The code prior to this change resulted in including both
the API and tests since the copy.include resulted as an
empty list by default since the stream is empty unless
explicitly configured.

related #52114
fixes #53183

buildSrc/src/main/java/org/elasticsearch/gradle/test/rest/CopyRestApiTask.java

@@ -51,7 +51,7 @@
  * @see RestResourcesPlugin
  */
 public class CopyRestApiTask extends DefaultTask {
-    private static final String COPY_TO = "rest-api-spec/api";
+    private static final String REST_API_PREFIX = "rest-api-spec/api";
     final ListProperty<String> includeCore = getProject().getObjects().listProperty(String.class);
     final ListProperty<String> includeXpack = getProject().getObjects().listProperty(String.class);
 
@@ -109,7 +109,7 @@ public FileTree getInputDir() {
 
     @OutputDirectory
     public File getOutputDir() {
-        return new File(getTestSourceSet().getOutput().getResourcesDir(), COPY_TO);
+        return new File(getTestSourceSet().getOutput().getResourcesDir(), REST_API_PREFIX);
     }
 
     @TaskAction
@@ -132,7 +132,13 @@ void copy() {
             project.copy(c -> {
                 c.from(project.zipTree(coreConfig.getSingleFile()));
                 c.into(getTestSourceSet().getOutput().getResourcesDir()); // this ends up as the same dir as outputDir
-                c.include(includeCore.get().stream().map(prefix -> COPY_TO + "/" + prefix + "*/**").collect(Collectors.toList()));
+                if (includeCore.get().isEmpty()) {
+                    c.include(REST_API_PREFIX + "/**");
+                } else {
+                    c.include(
+                        includeCore.get().stream().map(prefix -> REST_API_PREFIX + "/" + prefix + "*/**").collect(Collectors.toList())
+                    );
+                }
             });
         }
         // only copy x-pack specs if explicitly instructed

buildSrc/src/main/java/org/elasticsearch/gradle/test/rest/CopyRestTestsTask.java

@@ -48,7 +48,7 @@
  * @see RestResourcesPlugin
  */
 public class CopyRestTestsTask extends DefaultTask {
-    private static final String COPY_TO = "rest-api-spec/test";
+    private static final String REST_TEST_PREFIX = "rest-api-spec/test";
     final ListProperty<String> includeCore = getProject().getObjects().listProperty(String.class);
     final ListProperty<String> includeXpack = getProject().getObjects().listProperty(String.class);
 
@@ -103,7 +103,7 @@ public FileTree getInputDir() {
 
     @OutputDirectory
     public File getOutputDir() {
-        return new File(getTestSourceSet().getOutput().getResourcesDir(), COPY_TO);
+        return new File(getTestSourceSet().getOutput().getResourcesDir(), REST_TEST_PREFIX);
     }
 
     @TaskAction
@@ -128,7 +128,9 @@ void copy() {
                 project.copy(c -> {
                     c.from(project.zipTree(coreConfig.getSingleFile()));
                     c.into(getTestSourceSet().getOutput().getResourcesDir()); // this ends up as the same dir as outputDir
-                    c.include(includeCore.get().stream().map(prefix -> COPY_TO + "/" + prefix + "*/**").collect(Collectors.toList()));
+                    c.include(
+                        includeCore.get().stream().map(prefix -> REST_TEST_PREFIX + "/" + prefix + "*/**").collect(Collectors.toList())
+                    );
                 });
             }
         }