Supress flaky test, lib/mirrors/invocation_fuzz_test/string, on mac debug

tests/lib/lib.status

Index: tests/lib/lib.status
===================================================================
--- tests/lib/lib.status	(revision 41944)
+++ tests/lib/lib.status	(working copy)
@@ -118,15 +118,12 @@
 math/math_test: RuntimeError
 math/math2_test: RuntimeError
 
-[ $compiler == dart2js && ($jscl || $runtime == d8) && $unchecked ]
+[ $runtime == dart2js && $jscl ]
 mirrors/invocation_fuzz_test: RuntimeError # Issue 15566
 
-[ $compiler == dart2js && ($jscl || $runtime == d8) && $checked ]
-mirrors/invocation_fuzz_test/none: RuntimeError # Issue 15566
+[ $runtime == vm && $mode == debug && $system == macos ]
+mirrors/invocation_fuzz_test: Pass, RuntimeError # Issue 21707
 
-[ $compiler == dart2js && ($minified || $runtime == jsshell) ]
-mirrors/invocation_fuzz_test: RuntimeError # Issue 15566
-
 [ $compiler == dart2js && $runtime == jsshell ]
 async/future_test: RuntimeError # Timer interface not supported; dartbug.com/7728.
 async/slow_consumer2_test: RuntimeError # Timer interface not supported; dartbug.com/7728.