Add unit tests for Uri.IsWellFormedUri

Uri.IsWellFormedUri() reports a false negative when mixing characters
like Å or ตั together with any of the RFC 3986 section 2.2 Reserved
Characters, ! * ' ( ) ; : @ & = + $ , / ? # [ ].

This change adds a (failing) unit test for this bug.

Tests dotnet#21626

src/libraries/System.Private.Uri/tests/FunctionalTests/UriIsWellFormedUriStringTest.cs

@@ -117,6 +117,13 @@ public void UriIsWellFormed_AbsoluteWithPercentAndColonToRelative_AppendsDotSlas
             Assert.True(string.CompareOrdinal(rel.ToString(), 0, "./", 0, 2) == 0, "Cannot have colon in first segment, must append ./");
         }
 
+        [Fact]
+        public void UriIsWellFormed_MixedPercent()
+        {
+            Assert.True(Uri.IsWellFormedUriString("https://base.com/%C3%A5%2A"));
+            Assert.True(Uri.IsWellFormedUriString("https://base.com/%E0%B8%95%E0%B8%B1%2A"));
+        }
+
         [Fact]
         public void UriMakeRelative_ImplicitFileCommonBaseWithColon_AppendsDotSlash()
         {