[WebCryptoAPI] Defer harness completion

[jugglinmike]

This change ensures that all subtests are executed when the tests are
evaluated in dedicated worker contexts.

[foolip]

wpt-firefox-nightly-stability is failing on Taskcluster. There were only 4 affected tests and yet we got "Task timeout after 7200 seconds. Force killing container."

These are the actual tests that were run, after expanding multi-globals and variants:

/WebCryptoAPI/derive_bits_keys/ecdh_bits.https.any.html
/WebCryptoAPI/derive_bits_keys/ecdh_bits.https.any.worker.html
/WebCryptoAPI/derive_bits_keys/ecdh_keys.https.any.html
/WebCryptoAPI/derive_bits_keys/ecdh_keys.https.any.worker.html
/WebCryptoAPI/derive_bits_keys/hkdf.https.any.html?1-1000
/WebCryptoAPI/derive_bits_keys/hkdf.https.any.html?1001-2000
/WebCryptoAPI/derive_bits_keys/hkdf.https.any.html?2001-3000
/WebCryptoAPI/derive_bits_keys/hkdf.https.any.html?3001-last
/WebCryptoAPI/derive_bits_keys/hkdf.https.any.worker.html?1-1000
/WebCryptoAPI/derive_bits_keys/hkdf.https.any.worker.html?1001-2000
/WebCryptoAPI/derive_bits_keys/hkdf.https.any.worker.html?2001-3000
/WebCryptoAPI/derive_bits_keys/hkdf.https.any.worker.html?3001-last
/WebCryptoAPI/derive_bits_keys/pbkdf2.https.any.html?1-1000
/WebCryptoAPI/derive_bits_keys/pbkdf2.https.any.html?1001-2000
/WebCryptoAPI/derive_bits_keys/pbkdf2.https.any.html?2001-3000
/WebCryptoAPI/derive_bits_keys/pbkdf2.https.any.html?3001-4000
/WebCryptoAPI/derive_bits_keys/pbkdf2.https.any.html?4001-5000
/WebCryptoAPI/derive_bits_keys/pbkdf2.https.any.html?5001-6000
/WebCryptoAPI/derive_bits_keys/pbkdf2.https.any.html?6001-7000
/WebCryptoAPI/derive_bits_keys/pbkdf2.https.any.html?7001-8000
/WebCryptoAPI/derive_bits_keys/pbkdf2.https.any.html?8001-last
/WebCryptoAPI/derive_bits_keys/pbkdf2.https.any.worker.html?1-1000
/WebCryptoAPI/derive_bits_keys/pbkdf2.https.any.worker.html?1001-2000
/WebCryptoAPI/derive_bits_keys/pbkdf2.https.any.worker.html?2001-3000
/WebCryptoAPI/derive_bits_keys/pbkdf2.https.any.worker.html?3001-4000
/WebCryptoAPI/derive_bits_keys/pbkdf2.https.any.worker.html?4001-5000
/WebCryptoAPI/derive_bits_keys/pbkdf2.https.any.worker.html?5001-6000
/WebCryptoAPI/derive_bits_keys/pbkdf2.https.any.worker.html?6001-7000
/WebCryptoAPI/derive_bits_keys/pbkdf2.https.any.worker.html?7001-8000
/WebCryptoAPI/derive_bits_keys/pbkdf2.https.any.worker.html?8001-last

The live_backing.log.txt is a huge 975MB file with ~9.5 million lines...

@jugglinmike I don't think this is because of your changes at all, but can you file an issue about the stability job being unable to run these tests to completion? If it keeps coming up maybe we should fix it somehow.

[jugglinmike]

Sure, see gh-20052.

@jimsch: I'm able to merge this despite the CI failure, but I'd like to confirm with you before doing so. Does this patch look okay to you?

[jimsch]

I just did a re-review and I think that the patch will produce the same set of tests to be run. I would go ahead and integrate if you can live with the failures.

[jugglinmike]

Excellent. Thanks!