t/t0000-basic.sh: Run the passing TODO test inside its own test-lib

Change the passing TODO test in t0000-basic.sh to run inside its own
test-lib.sh. The motivation is to have nothing out of the ordinary on
a normal test run for test smoking purposes.

If every normal test run has a passing TODO you're more likely to turn
a blind eye to it and not to investigate cases where things really are
passing unexpectedly.

It also makes the prove(1) output less noisy. Before:

All tests successful.

Test Summary Report
-------------------
./t0000-basic.sh                                   (Wstat: 0 Tests: 46 Failed: 0)
TODO passed:   5
Files=484, Tests=6229, 143 wallclock secs ( 4.00 usr  4.15 sys + 104.77 cusr 351.57 csys = 464.49 CPU)
Result: PASS

And after:

All tests successful.
Files=484, Tests=6228, 139 wallclock secs ( 4.07 usr  4.25 sys + 104.54 cusr 350.85 csys = 463.71 CPU)
Result: PASS

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>