Add expected output tests for regex source generator (dotnet#80937)

* Add expected output tests for regex

In particular to help avoid significant formatting / readability regressions in the code output by the regex generator.

* Address PR feedback to normalize version

src/libraries/Common/tests/TestUtilities/System/AssertExtensions.cs

@@ -504,9 +504,21 @@ public static void AtLeastOneEquals<T>(T expected1, T expected2, T value)
         /// </summary>
         public static void Equal(string expected, string actual)
         {
-            if (!expected.Equals(actual))
+            try
             {
-                throw new AssertActualExpectedException(expected, actual, "Provided strings were not equal!");
+                Assert.Equal(expected, actual);
+            }
+            catch (Exception e)
+            {
+                throw new XunitException(
+                    e.Message + Environment.NewLine +
+                    Environment.NewLine +
+                    "Expected:" + Environment.NewLine +
+                    expected + Environment.NewLine +
+                    Environment.NewLine +
+                    "Actual:" + Environment.NewLine +
+                    actual + Environment.NewLine +
+                    Environment.NewLine);
             }
         }