Subtests created with async_test execute in parallel. Because each
subtest in the modified file use the same canvas element, parallel
execution make them susceptible to undesired interactions.

Declare the subtests using promise_test so that the harness executes
them in series.

@reillyeon requested this change in gh-20234, where they wrote

Since the generated async tests are reusing the same <canvas> element
should they be converted to promise tests so that they run sequentially
rather than attempting to apply a bunch of different constraints in parallel?
That seems to depend on whether or not captureStream() is guaranteed to
return fully independent MediaStream instances.

These tests do not pass on my system (neither from master nor from this branch), so I can't verify the validity of this change.