t0000: simplify HARNESS_ACTIVE hack

t0000: simplify HARNESS_ACTIVE hack

Commit 517cd55 set HARNESS_ACTIVE unconditionally in
sub-tests, because that value affects the output of
"--verbose". t0000 needs stable output from its sub-tests,
and we may or may not be running under a TAP harness.

That commit made the decision to always set the variable,
since it has another useful side effect, which is
suppressing writes to t/test-results by the sub-tests (which
would just pollute the real results).

Since the last commit, though, the sub-tests have their own
test-results directories, so this is no longer an issue. We
can now update a few comments that are no longer accurate
nor necessary.

We can also revisit the choice of HARNESS_ACTIVE. Since we
must choose one value for stability, it's probably saner to
have it off. This means that future patches could test
things like the test-results writing, or the "--quiet"
option, which is currently ignored when run under a harness.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>