Now run-tests.py understands "suite/foo/bar" with forward slashes for
command-line test selection on all test suites on all platforms.

Previously, file-based suites like mjsunit also accepted "mjsunit/foo\bar";
that behavior is sacrificed here in favor of unification. For the cctest
suite, OTOH, it wasn't possible on Windows to select specific tests at all.

Review URL: https://codereview.chromium.org/1348653003

Cr-Commit-Position: refs/heads/master@{#30794}