t/Makefile: pass test opts to valgrind target properly

t/Makefile: pass test opts to valgrind target properly

The valgrind target just reinvokes make with GIT_TEST_OPTS
set to "--valgrind". However, it does this using an
environment variable, which means GIT_TEST_OPTS in your
config.mak would override it, and "make valgrind" would
simply run the test suite without valgrind on.

Instead, we should pass GIT_TEST_OPTS on the command-line,
overriding what's in config.mak, and take care to append to
whatever the user has there already.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>