[TSan][libdispatch] Disable test to unbreak build

This test requires the %link_libcxx_tsan substitution, which was
recently added and is not available in swift-5.1-branch.

Author: yln

test/tsan/libdispatch/groups-destructor.cc

@@ -1,6 +1,10 @@
 // RUN: %clangxx_tsan %s %link_libcxx_tsan -o %t
 // RUN: %run %t 2>&1 | FileCheck %s --implicit-check-not='ThreadSanitizer'
 
+// This test requires the %link_libcxx_tsan substitution, which was recently
+// added and is not available in swift-5.1-branch.
+// REQUIRES: link_libcxx_tsan
+
 #include <dispatch/dispatch.h>
 
 #include <memory>