[devops] Return a non-zero exit code if tests fail on older macOS versions. (#12937)

Return a non-zero exit code if tests fail on older macOS versions, but keep
running tests. This way the step shows up as orange if something fails (and
not green, which is confusing).

tools/devops/automation/templates/mac/build.yml

@@ -199,7 +199,7 @@ steps:
           $msg.AppendLine("* $test")
       }
       $request = New-GitHubComment -Header "Tests failed on macOS $Env:CONTEXT" -Description "Tests failed on $Env:CONTEXT." -Message $msg.ToString() -Emoji ":x:"
-
+      exit 1
     } else {
       Set-GitHubStatus -Status "success" -Description "Tests passed Xamarin.Mac tests on macOS $Env:CONTEXT passed." -Context "$Env:CONTEXT"
       $request = New-GitHubComment -Header "Tests passed on macOS $Env:CONTEXT" -Description "Tests passed" -Message "**All** tests on macOS X $Env:CONTEXT passed." -Emoji ":white_check_mark:"
@@ -211,3 +211,4 @@ steps:
     GITHUB_TOKEN: $(GitHub.Token)
     SYSTEM_ACCESSTOKEN: $(System.AccessToken)
     MONO_DEBUG: no-gdb-backtrace
+    continueOnError: true