check: small cleanups and refactorings in test harness and drivers

check: small cleanups and refactorings in test harness and drivers

* lib/tap-driver (yn): New subroutine, converts a boolean value to
a "yes" or "no" string.
(write_test_results): Use it, and related reformatting.
* lib/test-driver (trap): Also remove the `.trs' file on signals,
for extra safety.
(fatal): Remove this function, it's never used.
($logfile, $trsfile): Renamed ...
($log_file, $trs_file): ... to these, for clarity, and in order to
be more consistent with the `tap-driver' script.
Improve a couple of comments.
* automake.in (handle_tests): Don't define anymore the now-obsolete
make macro `$(TEST_LOGS_TMP)', nor add it to the list of files to
be removed upon "make mostlyclean".
* lib/am/check.am ($(TEST_SUITE_LOG)): New shell function `f_ok',
tells whether a path refers to an existing, regular, readable file.
Use it throughout.
(recheck): Be safer w.r.t. make implementation that run recipes
with `errexit' shell flag active.