tests: add Valgrind support

Now running tests with the "--valgrind" option will run cgit under
Valgrind instead of all Git commands.

Signed-off-by: John Keeping <john@keeping.me.uk>
tests: add CGIT_TEST_OPTS variable to Makefile

This allows running the entire test suite with a set of command-line
options.  For example:

	make test CGIT_TEST_OPTS=--valgrind

Signed-off-by: John Keeping <john@keeping.me.uk>