test-lib: verbose mode for only tests matching a pattern

With the new --verbose-only=<pattern> option, one can enable --verbose
at a per-test granularity.  The pattern is matched against the test
number, e.g.

  ./t0000-basic.sh --verbose-only='2[0-2]'

to see only the full output of test 20-22, while showing the rest in the
one-liner format.

This is arguably not *too* useful on its own, but makes the next patch
easier to follow.

Signed-off-by: Thomas Rast <trast@inf.ethz.ch>
test-lib: valgrind for only tests matching a pattern

With the new --valgrind-only=<pattern> option, one can enable
--valgrind at a per-test granularity, exactly analogous to
--verbose-only from the previous commit.

The options are wired such that --valgrind implies --verbose (as
before), but --valgrind-only=<pattern> implies
--verbose-only=<pattern> unless --verbose is also in effect.

Signed-off-by: Thomas Rast <trast@inf.ethz.ch>
test-lib: allow prefixing a custom string before "ok N" etc.

This is not really meant for external use, but allows the next commit
to neatly distinguish between sub-tests and the main run.

Signed-off-by: Thomas Rast <trast@inf.ethz.ch>
test-lib: support running tests under valgrind in parallel

With the new --valgrind-parallel=<n> option, we support running the
tests in a single test script under valgrind in parallel using 'n'
processes.

This really follows the dumbest approach possible, as follows:

* We spawn the test script 'n' times, using a throw-away
  TEST_OUTPUT_DIRECTORY.  Each of the instances is given options that
  ensures that it only runs every n-th test under valgrind, but
  together they cover the entire range.

* We add up the numbers from the individual tests, and provide the
  usual output.

This is really a gross hack at this point, and should be improved.  In
particular we should keep the actual outputs somewhere more easily
discoverable, and summarize them to the user.

Nevertheless, this is already workable and gives a speedup of more
than 2 on a dual-core (hyperthreaded) machine, using n=4.  This is
expected since the overhead of valgrind is so big (on the order of 20x
under good conditions, and a large startup overhead at every git
invocation) that redundantly running the non-valgrind tests in between
is not that expensive.

Signed-off-by: Thomas Rast <trast@inf.ethz.ch>