[Toybox] Kernighan's awk repo

[Andy Chu]

Update on this repo: I added HTML coverage reports, and the results are here:

https://github.com/andychu/bwk/blob/master/test-results/coverage.md

So the 5 test suites together have 91% line coverage and 79% branch
coverage.  And it breaks it down by file and shows individual lines.
Note that coverage is another build variant like ASAN.

LLVM apparently has 2 different coverage modes: one which is
compatible with gcc, and then the "sanitizer coverage" mode.  The
latter is supposed to cause less lock contention on counters, but that
doesn't matter for awk or toybox since there are no threads.  I just
used whatever seemed to be easiest to set up for now.

(Also the usage of test.sh changed, which is documented in README.md)

Andy