Disable flaky generators-debug-scopes test

This test is flaky when run with --optimize-for-size.

BUG=v8:5950

Review-Url: https://codereview.chromium.org/2689603003
Cr-Commit-Position: refs/heads/master@{#43097}

test/debugger/debugger.status

@@ -26,6 +26,9 @@
 
   # Too slow in debug mode and on slow platforms.
   'regress/regress-2318': [PASS, ['mode == debug or (arch != ia32 and arch != x64) or asan == True or msan == True', SKIP]],
+
+  # Issue 5950: Flaky with --optimize-for-size.
+  'debug/es6/generators-debug-scopes': [PASS, ['system == linux and arch == x64', SKIP]],
 }],  # ALWAYS
 
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