Add support for --force parameter to run-performance-tests

Add support for --force parameter to run-performance-tests
https://bugs.webkit.org/show_bug.cgi?id=90279

Reviewed by Dirk Pranke.

It's helpful to be able to run tests from the Skipped list of the performance tests.

* Scripts/webkitpy/performance_tests/perftestsrunner.py:
(PerfTestsRunner._parse_args):
(PerfTestsRunner._collect_tests):
* Scripts/webkitpy/performance_tests/perftestsrunner_unittest.py: Add test.
(test_collect_tests_with_skipped_list):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@121571 268f45cc-cd09-0410-ab3c-d52691b4dbfc