t0000: disable GIT_TEST_FAIL_PREREQS in sub-tests

t0000: disable GIT_TEST_FAIL_PREREQS in sub-tests

The test added by 477dcaddb6 (tests: do not let lazy prereqs inside
`test_expect_*` turn off tracing, 2020-03-26) runs a sub-test script
that traces a test with a lazy prereq, like:

  test_have_prereq LAZY && echo trace

That won't work if GIT_TEST_FAIL_PREREQS is set in the environment,
because our have_prereq will report failure, and we won't run the echo
at all.

We could work around this by avoiding the &&-chain, but we can
fix this and any future tests at once by unsetting that variable for our
sub-tests. These are meant to be controlled environments where we test
the test-suite itself; the outer test snippet should be in charge of the
sub-test environment, not whatever mode the user happens to be running
in.

Reported-by: Son Luong Ngoc <sluongng@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>