Valgrind testing is useful, but really should be done as a separate
exercise from the usual regression testing, as it takes way too long.

Rather than including it by default in all.tests and making people
exclude it with the -x valgrind option (or by using quick.tests), it
makes sense to explicitly request valgrind testing with --valgrind.

To perform valgrind testing:
$ piglit-run.py --valgrind <options> tests/quick.tests results/vg-1

The ExecTest class now handles Valgrind wrapping natively, rather than
relying on the tests/valgrind-test/valgrind-test shell script wrapper.
This provides a huge benefit: we can leverage the interpretResult()
function to make it work properly for any subclass of ExecTest.  The
old shell script only worked for PlainExecTest (piglit) and GleanTest.

Another small benefit is that you can now use --valgrind with any test
profile (such as quick.tests).  Also, you can use all.tests without
having to remember to specify "-x valgrind".

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>