parse --windows-only func test arg on Windows only

The --windows-only functional test command-line option causes
only tests in the WindowsOnly category to be run; since this
makes sense on Windows exclusively, we move the parsing of this
command-line argument into a Windows-only block, so it will be
ignored on other platforms.

Scalar.FunctionalTests/Program.cs

@@ -76,22 +76,22 @@ public static void Main(string[] args)
                 ScalarTestConfig.FileSystemRunners = FileSystemRunners.FileSystemRunner.DefaultRunners;
             }
 
-            if (runner.HasCustomArg("--windows-only"))
-            {
-                includeCategories.Add(Categories.WindowsOnly);
-
-                // RunTests unions all includeCategories.  Remove ExtraCoverage to
-                // ensure that we only run tests flagged as WindowsOnly
-                includeCategories.Remove(Categories.ExtraCoverage);
-            }
-
             if (RuntimeInformation.IsOSPlatform(OSPlatform.OSX))
             {
                 excludeCategories.Add(Categories.MacTODO.TestNeedsToLockFile);
                 excludeCategories.Add(Categories.WindowsOnly);
             }
             else
             {
+                if (runner.HasCustomArg("--windows-only"))
+                {
+                    includeCategories.Add(Categories.WindowsOnly);
+
+                    // RunTests unions all includeCategories.  Remove ExtraCoverage to
+                    // ensure that we only run tests flagged as WindowsOnly
+                    includeCategories.Remove(Categories.ExtraCoverage);
+                }
+
                 excludeCategories.Add(Categories.MacOnly);
             }