build: use Automake's new parallel-tests feature

build: use Automake's new parallel-tests feature

* build-aux/check.mk: Remove, not needed any more.
* tests/Makefile.am (AUTOMAKE_OPTIONS): Add parallel-tests and
color-tests.
* tests/check.mk: Do not include build-aux/check.mk any more.
(SUFFIXES, TEST_LOGS): Remove.
(TESTS_ENVIRONMENT): Use $$f rather than $$tst in $PERL invocation.
* gnulib-tests/Makefile.am: Do not include build-aux/check.mk.
(AUTOMAKE_OPTIONS): New macro, add parallel-test and color-tests.
(TEST_LOGS): Remove.