Fix history_sidebar_forget_site_verify_if_remembered test case

[spronoza]

Fix history_sidebar_delete_site_verify_if_remembered test case #3294

[pavelciapa]

Fails on osx

        expected_4 = exists(history_today_sidebar_pattern, 10)
>       assert expected_4 is True, 'Expand history button displayed properly.'
E       AssertionError: Expand history button displayed properly.
E       assert False is True

tests/firefox/history/history_sidebar_forget_site_verify_if_remembered.py:53: AssertionError

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[pavelciapa]

Approved

[tracywalker]

The try/except block at lines 69-73 is not necessary around an exists statement. However, Why was the wait_vanish() (which does require being wrapped in a try/except block) removed?

Also, I just filed issue #3723 on reviewing tests using asserting is not True for exists(pattern).

[spronoza]

The try/except block at lines 69-73 is not necessary around an exists statement. However, Why was the wait_vanish() (which does require being wrapped in a try/except block) removed?

Also, I just filed issue #3723 on reviewing tests using asserting is not True for exists(pattern).

The wait_vanish is wasn't able to work with string. The "try/except" is just have been forgotten to be removed after replacing it on "exists". My fault