run-webkit-tests calls "nm" when it doesn't need to

run-webkit-tests calls "nm" when it doesn't need to
https://bugs.webkit.org/show_bug.cgi?id=77143

Reviewed by Dirk Pranke.

If we are not running any test that can be skipped by checking
the DRT feature list or looking at the list of symbols, avoid calling
out to DRT or calling "nm".

This saves ~700ms on my Mac Pro. In cases where you still have to call out
to "nm" it's only a couple ms slower.

* Scripts/webkitpy/layout_tests/models/test_expectations.py:
(TestExpectations.__init__):
* Scripts/webkitpy/layout_tests/port/base.py:
(Port.skipped_tests):
* Scripts/webkitpy/layout_tests/port/webkit.py:
(WebKitPort._should_check_for_missing_features):
(WebKitPort):
(WebKitPort._should_check_for_missing_symbols):
(WebKitPort._skipped_tests_for_unsupported_features):
(WebKitPort.skipped_layout_tests):
(WebKitPort.skipped_tests):
* Scripts/webkitpy/layout_tests/port/webkit_unittest.py:
(WebKitPortTest.test_skipped_directories_for_symbols):
(test_skipped_directories_for_features):
(test_skipped_directories_for_features_no_matching_tests_in_test_list):
(test_skipped_layout_tests):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@106058 268f45cc-cd09-0410-ab3c-d52691b4dbfc