Revert "RBS: Pathname#taint and Pathname#untaint are removed"

This reverts commit 46132e78b7438389bf49a860a0cd8957f685dcb3, in favor
of 298cb57c11.

tool/rbs_skip_tests

@@ -26,5 +26,3 @@ NetInstanceTest depending on external resources
 TestHTTPRequest depending on external resources
 TestSingletonNetHTTPResponse depending on external resources
 TestInstanceNetHTTPResponse depending on external resources
-test_taint(PathnameInstanceTest) removed at ruby/pathname@c52fd3a835
-test_untaint(PathnameInstanceTest) removed at ruby/pathname@c52fd3a835