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>