The first step is probably making an issue in upstream WPT to discuss.

Agreed, this sounds like it may be good idea, and should be discussed in an upstream issue.

Is the reason for this that long subtest names are an issue for App Engine datastore if subtest names are used as StringProperty or used in keys? There's probably some way that we could work around that if we don't end up putting a limit on subtest names. The simplest workaround might be to truncate very long subtest names.

Yes, originally it was motivated by not wanting to break the 1MB limit for datastore objects. However, the design doc has changed a bit and I'm not planning on storing test results in Datastore, so this is no longer a pressing issue. foolip seemed to be interested in limiting subtest name length in general just as a good practice.

The first thing to do is probably generate a histogram of subtest name lengths. I'll update the TODOs.

Should we WontFix this issue perhaps? I can't fine this issue references in the Chromium code, are there TODOs elsewhere pointing here to update?

I wouldn't mind marking WontFix. If we want to pursue this later, as #1 says, this would be a feature request on the wpt GitHub issue tracker.