Reviewed by Eric Seidel.

        Rewrite new-run-webkit-test's wait_for_threads_to_finish loop to
        check for exceptions on all threads, not just the first thread.

        This change also changes the logging behavior for wedged threads
        to only dump the stacks of threads that are actually wedged.

        Refactor the thread classes in the dump_render_tree_thread module
        to make the contract between TestRunner and TestShellThread clearer.

        Added a bunch of unit tests.
        https://bugs.webkit.org/show_bug.cgi?id=38561

        * Scripts/webkitpy/layout_tests/layout_package/dump_render_tree_thread.py:
        * Scripts/webkitpy/layout_tests/run_webkit_tests.py:
        * Scripts/webkitpy/layout_tests/run_webkit_tests_unittest.py:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@65343 268f45cc-cd09-0410-ab3c-d52691b4dbfc