perf tests: reorganize
* t/list-of-tests.mk (handwritten_TESTS): Don't list ($perf_TESTS) in
here anymore.
* Makefile.am (EXTRA_DIST): Add them explicitly.
(perf): New target, run the performance tests and save the logs of
the ones with unexpected results in the file 't/perf/test-suite.log',
which is thus ...
(PERF_TEST_SUITE_LOG): ... defined in this variable ...
(CLEANFILES): ... and added to this.
(test_subdirs): Remove 't/perf': the tests in there are no more
listed in $(TESTS).
* t/ax/test-init.sh: Don't explicitly skip "perf" test here.
Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>