tap/awk: handle non-zero exit status from the test command

tap/awk: handle non-zero exit status from the test command

* lib/tap-driver.sh (get_test_exit_message): New function in the
awk script, used to extract the exit status of the test program,
or at least a good guess of it.
(write_test_results): Use it, reporting an ERROR result if it
detects that the test program exited with a non-zero status.
* tests/tap-signal.test: Account for the differences in the
error messages generated by the awk and perl TAP drivers.  Fix
an unrelated typo in comments since we are at it.