test harness: work around a VPATH rewrite issue

test harness: work around a VPATH rewrite issue

* lib/am/check.am (am__TEST_BASES): Removed, it's role taken
over by ...
(am__set_TESTS_bases): ... these new variable.
($(TEST_SUITE_LOG): Use it, to avoid VPATH rewrite issues.
* automake.in (handle_tests): Update the code for the cleanup
of the `.trs' file to use `$(TEST_LOGS)' instead of
`$(am__TEST_BASES)'.
* tests/test-harness-vpath-rewrite.test: New test, better
exposing the problem fixed by this change
* tests/Makefile.am (TESTS): Add it.
* tests/test-trs-basic.test: Update and extend.