perf: beginning of a performance testsuite

perf: beginning of a performance testsuite

Some tests in the Automake testsuite already aims only at verifying
the performance, rather than the correctness, of some operations.
Still, they are somewhat shoehorned and forced into the PASS/FAIL
framework (say, with the 'ulimit' shell builtin used to verify some
operation doesn't take up too much time or memory), but that is
conceptually a stretch, and has already caused problems in practice
(see automake bug#11512 for an example).

So we start moving the "performance tests" out of the testsuite proper,
and make them run only "on demand" (when the user exports the variable
'AM_TESTSUITE_PERF' to "yes").  Ideally, we should provide those tests
with a custom runner/driver that measures and displays the relevant
performance information, but doing that correctly and with the right
APIs is definitely more difficult, so we leave it for a later step
(an hope we'll take such a step eventually).

* t/cond29.sh: Move ...
* t/perf/cond.sh: ... here, and adjust.
* t/testsuite-recheck-speed.sh: Move ...
* t/perf/testsuite-recheck.sh: ... here.
* t/testsuite-summary-speed.sh: Move ...
* t/perf/testsuite-summary.sh: ... here.
* t/list-of-tests.mk (perf_TESTS): New variable, listing the tests in
the 't/perf' directory.
(handwritten_TESTS): Adjust.
* defs: Skip any tests in the 't/perf/' subdirectory unless the
'AM_TESTSUITE_PERF' variable is set to "yes" or "y".
* .gitignore: Update.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>