More explicitly label `.tentative` tests

[saschanaz]

One should be able to see it's not in the spec by seeing .tentative somewhere in the test file name, but it's not super visible:

• my-amazing-web-platform-test.tentative.html - reasonably visible I guess?
• my-really-amazing-web-platform-test.https.tentative.any.serviceworker.html - well, it's there in a pile of other modifiers...

One could also just highlight the word by Control+F, but maybe it would be better it's more explictly labeled or highlighted by default.

(And maybe exclude it while counting total test count? 🤔)

[zcorpan]

There's a search query if you want to count or exclude from the count:

• is:tentative
• is:!tentative

But a visible by default indicator (a separate column?) seems nice.