Report subtests through the exception handlers

[jelmer]

Subtest failures were buffered on the test case and replayed at the end of RunTest._run_core, bypassing testtools' own exception machinery. That lost several behaviours relative to a non-subtest failure:

• a subclass overriding skipException had its skip reported as an error, since subTest caught the hardcoded unittest.SkipTest
• expectFailure inside a subtest leaked the private _ExpectedFailure as an error instead of recording an expected failure
• MultipleExceptions was reported verbatim rather than unpacked
• details attached inside a failing subtest never reached the result
• AsynchronousDeferredRunTest._run_core never drained the buffers, so a failing subtest reported as a pass

Report each subtest as it completes instead, dispatching on the case's exception handlers and passing the details accumulated inside the block. Since reporting no longer lives in _run_core, the Twisted runner is covered too; it only needs to know not to also report success.

Also align with unittest where the buffering had diverged.

Follow-up to #630.