[streams] Use promise_test in web-platform-tests. (#400)

Use promise_test in cancel.js and general.js.
As promise_tests are executed sequentially, this change shortens the timeouts
in tests.
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/streams/pull/400

-- Commit Summary --

  * Use promise_test in web-platform-tests.

-- File Changes --

    M reference-implementation/web-platform-tests/readable-streams/cancel.js (224)
    M reference-implementation/web-platform-tests/readable-streams/general.js (529)
    M reference-implementation/web-platform-tests/resources/rs-utils.js (4)

-- Patch Links --

https://github.com/whatwg/streams/pull/400.patch
https://github.com/whatwg/streams/pull/400.diff

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https://github.com/whatwg/streams/pull/400