(chocolatey#1759) Add test for new enhanced exit code

Now, when you have useEnhancedExitCodes turned on, if you try to
add an apikey when it is already added the exit code will change to be
a 2, giving a clear indication that no action was taken.

This commit adds a Pester test to verify that this works as expected.

tests/pester-tests/commands/choco-apikey.Tests.ps1

@@ -371,6 +371,41 @@ Describe "choco <_>" -ForEach $Command -Tag Chocolatey, ApiKeyCommand {
         }
     }
 
+    Context "Adding an apikey when it is already added" {
+        BeforeAll {
+            Restore-ChocolateyInstallSnapshot
+
+            # Ensure that the apikey is indeed set
+            $null = Invoke-Choco apikey add --source "https://somewhere.out/there/" --api-key "123-4567-89"
+
+            $Output = Invoke-Choco apikey add --source "https://somewhere.out/there/" --api-key "123-4567-89"
+        }
+
+        It "Exits with ExitCode 0" {
+            $Output.ExitCode | Should -Be 0 -Because $Output.String
+        }
+
+        It "Changes Nothing" {
+            $Output.Lines | Should -Contain "Nothing to change. Config already set."
+        }
+
+        Context "when using enhanced exit codes" {
+            BeforeAll {
+                $null = Enable-ChocolateyFeature -Name "useEnhancedExitCodes"
+
+                $Output = Invoke-Choco apikey add --source "https://somewhere.out/there/" --api-key "123-4567-89"
+            }
+
+            It "Exits with ExitCode 2" {
+                $Output.ExitCode | Should -Be 2 -Because $Output.String
+            }
+
+            It "Changes Nothing" {
+                $Output.Lines | Should -Contain "Nothing to change. Config already set."
+            }
+        }
+    }
+
     # This needs to be the last test in this block, to ensure NuGet configurations aren't being created.
     Test-NuGetPaths
 }