swift234

StdlibUnittest: run tests out of process

The test harness now can recover after test crashes, allowing:

- check for crashes themselves (without reporting them to the Python lit driver,
  which is about 10x slower -- even if CrashTracer is disabled);

- recover from unexpected test crashes and run the rest of the tests;

- this lays the groundwork for assertions that end the test execution, but
  allow the rest of the tests to run (rdar://17906801).

Note that we don't spawn a fresh process for every test.  We create a child
process and reuse it until it crashes.


Swift SVN r21090