This allows functions which use KJ_EXPECT and other test macros to be built without linking libkj-test, which pulls in a possibly-unwanted main() function.

In particular, afl-testcase.c++ could not link in -fno-exceptions mode without this change.

Warned by gcc 5.3.1

- Reduce noise.
- Trace through async callbacks.

Fix bug in test filters where if you specified only a single test rather than a range, all tests after it in the same file would run.

A compatibility layer is provided to ease migration.