Include C# 9 tests in CI builds

Author: sharwell

build/opencover-report.ps1

@@ -41,6 +41,7 @@ $report_folder = '.\OpenCover.Reports'
 $target_dll = "..\StyleCop.Analyzers\StyleCop.Analyzers.Test\bin\$Configuration\net452\StyleCop.Analyzers.Test.dll"
 $target_dll_csharp7 = "..\StyleCop.Analyzers\StyleCop.Analyzers.Test.CSharp7\bin\$Configuration\net46\StyleCop.Analyzers.Test.CSharp7.dll"
 $target_dll_csharp8 = "..\StyleCop.Analyzers\StyleCop.Analyzers.Test.CSharp8\bin\$Configuration\net472\StyleCop.Analyzers.Test.CSharp8.dll"
+$target_dll_csharp9 = "..\StyleCop.Analyzers\StyleCop.Analyzers.Test.CSharp9\bin\$Configuration\net472\StyleCop.Analyzers.Test.CSharp9.dll"
 
 If (Test-Path $report_folder) {
 	Remove-Item -Recurse -Force $report_folder
@@ -111,6 +112,24 @@ If (($AppVeyor -or $Azure) -and -not $?) {
 	$exitCode = $LASTEXITCODE
 }
 
+&$opencover_console `
+	-register:$register_mode `
+	-threshold:1 -oldStyle `
+	-returntargetcode `
+	-hideskipped:All `
+	-filter:"+[StyleCop*]*" `
+	-excludebyattribute:*.ExcludeFromCodeCoverage* `
+	-excludebyfile:*\*Designer.cs `
+	-output:"$report_folder\OpenCover.StyleCopAnalyzers.xml" `
+	-mergebyhash -mergeoutput `
+	-target:"$xunit_runner_console_net472" `
+	-targetargs:"$target_dll_csharp9 -noshadow $AppVeyorArg -xml StyleCopAnalyzers.CSharp9.xunit.xml"
+
+If (($AppVeyor -or $Azure) -and -not $?) {
+	$host.UI.WriteErrorLine('Build failed; coverage analysis may be incomplete.')
+	$exitCode = $LASTEXITCODE
+}
+
 If (-not $NoReport) {
 	&$report_generator -targetdir:$report_folder -reports:$report_folder\OpenCover.*.xml
 	$host.UI.WriteLine("Open $report_folder\index.htm to see code coverage results.")