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>