Run test on Linux and Mac

src/Framework/AspNetCoreAnalyzers/test/Http/HeaderDictionaryAddTest.cs

@@ -147,22 +147,25 @@ public static IEnumerable<object[]> FixedWithAppendAddsUsingDirectiveTestData()
         };
     }
 
-    [ConditionalTheory]
-    [OSSkipCondition(OperatingSystems.Linux)]
-    [OSSkipCondition(OperatingSystems.MacOSX)]
+    [Theory]
     [MemberData(nameof(FixedWithAppendAddsUsingDirectiveTestData))]
     public async Task IHeaderDictionary_WithAdd_FixedWithAppend_AddsUsingDirective(string source, string fixedSource)
     {
+        // Source is cloned on Windows with CRLF line endings, then the test is run by Helix in Windows/Linux/macOS.
+        // When Roslyn adds a new `using`, it gets added followed by Environment.NewLine.
+        // For Linux/macOS, the actual result is `\n`, however, the source is cloned on Windows with CRLF expectation.
+        // We replace all line endings with Environment.NewLine to avoid this.
+
         // Arrange & Act & Assert
         await VerifyCS.VerifyCodeFixAsync(
-            source.TrimStart(),
+            source.TrimStart().ReplaceLineEndings(),
             new[]
             {
                 new DiagnosticResult(DiagnosticDescriptors.DoNotUseIHeaderDictionaryAdd)
                     .WithLocation(0)
                     .WithMessage(Resources.Analyzer_HeaderDictionaryAdd_Message)
             },
-            fixedSource.TrimStart(),
+            fixedSource.TrimStart().ReplaceLineEndings(),
             codeActionEquivalenceKey: AppendCodeActionEquivalenceKey);
     }