coverage: set DEFAULT_TEST_TARGET to avoid using prove

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>