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>