✅ Skip flaky shadow integration test on Safari

[rsimha]

This is the most common failure cause on master today.

DESCRIBE => AMP shadow v0
  IT => "before each" hook for "should attach shadow AMP document"
    ✗ Timeout of 10000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves.
  IT => "before each" hook for "should attach shadow AMP document"
    ✗ Timeout of 10000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves.

Safari 11.1.2 (Mac OS X 10.13.6): Executed 91 of 462 (1 FAILED) (skipped 371) (4 mins 10.788 secs / 1 min 37.474 secs)
Safari 12.1.1 (Mac OS X 10.13.6): Executed 91 of 462 (1 FAILED) (skipped 371) (4 mins 11.602 secs / 1 min 37.873 secs)

Related to #26863

[rsimha]

All tests green on Safari: https://travis-ci.org/ampproject/amphtml/jobs/652641725#L2370-L2374