update-backlog: promote subtests when full test promotion isn't possible

[ErichDonGubler]

@nical and I have observed that there are quite a few tests—particularly tests for parsing and parameterized API tests not associated with textures specifically—that actually run within the wptrunner timeout window on Firefox, but have a small subset of tests that do not pass. We're not sure if those subcases are trivial to resolve. We can, however, cut the Gordian knot by empowering ourselves to promote the subtests that do perma-PASS, get them locked into tier 2, and move on with other triage.

[ErichDonGubler]

Estimate: M (~3 days; ~2 days to implement, 1 to triage affected tests).

[ErichDonGubler]

I'm guessing we'll yield a couple dozen promoted tests, with many subtests promoted. We will want to start representing our tier promotion progress with test and subtest counts from the point that we do this.