coverage: add performance tests on some parallel-tests aspects

coverage: add performance tests on some parallel-tests aspects

* t/testsuite-recheck-speed.sh: New test.
* t/testsuite-summary-speed.sh: Likewise.
* t/list-of-tests.mk: Update.

The new tests are not meant to PASS/FAIL, but rather to help us to
obtain quantitative measurements of the performance improvements
offered by soon-to-appear optimization patches.

Timing before the optimizations implemented by following patches:

  + Slow Debian i686 system, 1 core x 1.5 GHz, 768 MB of RAM,
    GNU make 3.81:
     - testsuite-recheck-speed.sh: 1 run, ~ 6 minutes
     - testsuite-summary-speed.sh: 1 run, ~ 4 minutes

  + Fast Solaris 10 i686 system, 4 cores x 3 GHz, 20 GB of RAM,
    Solaris CCS make:
     - testsuite-recheck-speed.sh: 4 runs, ~ 3 minutes
     - testsuite-summary-speed.sh: 2 runs, ~ 6 minutes

  + Very fast Fedora ppc64 system, 64 cores x 3.5 GHz, 64 GB of RAM,
    GNU make 3.82:
     - testsuite-recheck-speed.sh: 5 runs, ~ 4 minutes
     - testsuite-summary-speed.sh: 5 runs, ~ 7 minutes

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