storage/estimate-usage-details-application-cache.https.tentative.html is "flaky" (fails ./wpt run --verify)

[foolip]

#14531 adds the test /storage/estimate-usage-details-application-cache.https.tentative.html which was failing the Taskcluster stability check using ./wpt run --verify):

Subtest	Results	Messages
	OK
estimate() shows usage increase after app is cached	FAIL: 5/10, PASS: 5/10	assert_greater_than: expected a number greater than 3364 but got 3364

I confirmed this locally in #14531 (comment).

The test isn't flaky in the sense of randomly failing, but it involves browser-global state, and ./wpt run --verify appears to run the test in such a way that the global state is shared across test invocations.

This could possibly be fixed in a manner similar to #15018.

[foolip]

@jgraham have you seen cases like this with Firefox as well? Will tests that have this problem ever run into trouble in Mozilla's CI setup? What I'm wondering is whether it's too high a bar to require tests to involve no global state. I don't know the details of the ./wpt run --verify scheduling, but I guess the tests are run in sequence at least, never in parallel? (Parallel would make it very hard to write fullscreen tests.)