coverage: set DEFAULT_TEST_TARGET to avoid using prove

If the user sets DEFAULT_TEST_TARGET=prove in his config.mak, that
carries over into the coverage tests.  Which is really bad if he also
sets GIT_PROVE_OPTS=-j<..> as that completely breaks the coverage
runs.

Instead of attempting to mess with the GIT_PROVE_OPTS, just force the
test target to 'test' so that we run under make, like we intended all
along.

Signed-off-by: Thomas Rast <trast@inf.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -2551,7 +2551,7 @@ coverage-compile:
 
 coverage-test: coverage-clean-results coverage-compile
 	$(MAKE) CFLAGS="$(COVERAGE_CFLAGS)" LDFLAGS="$(COVERAGE_LDFLAGS)" \
-		-j1 test
+		DEFAULT_TEST_TARGET=test -j1 test
 
 coverage-report:
 	$(QUIET_GCOV)for dir in $(object_dirs); do \