Bug 1591649 [wpt PR 19915] - [mediacapture-streams] Refactor test, a=testonly

Bug 1591649 [wpt PR 19915] - [mediacapture-streams] Refactor test, a=testonly

Automatic update from web-platform-tests
[mediacapture-streams] Refactor test (#19915)

testharness.js was recently extended with an API to explicitly opt-in to
the "single page test" feature [1]. As per WPT RFC 28 [2], tests which
do not use this API and which do not declare any subtests will soon be
reported as a harness error.

Update the tests which previously opted in implicitly to instead declare
a single subtest (so that it is no longer a single-page test).

[1] https://github.com/web-platform-tests/wpt/pull/19449
[2] https://github.com/web-platform-tests/rfcs/blob/master/rfcs/single_test.md
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wpt-commits: 3ce3017ee8b575af6a0e226f54d1bda4e241838d
wpt-pr: 19915

Differential Revision: https://phabricator.services.mozilla.com/D53507