test-lib: self-test that --verbose works

test-lib: self-test that --verbose works

t0000 contains some light self-tests of test-lib.sh, but --verbose was
not covered.  Add a test.

The only catch is that the presence of a test harness influences the
output (specifically, the presence of some empty lines).  So we need
to unset TEST_HARNESS or set it to a known value.  Leaving it unset
leads to spurious test failures in the final summary, which come from
the subtest.  So we always set it.

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