Move promise_test and assert_promise_rejects out of worker-test-harness.

These methods have nothing to do with service worker based tests, and are just
as useful in normal testharness based tests, so move them to a separate file.

Additionally rename worker-test-harness.js to worker-testharness.js for consistency.

Review URL: https://codereview.chromium.org/647493002

git-svn-id: svn://svn.chromium.org/blink/trunk@183557 bbb929c8-8fbe-4397-9dbb-9b2b20218538