t0000: fix test if run with TEST_OUTPUT_DIRECTORY

t0000: fix test if run with TEST_OUTPUT_DIRECTORY

Testcases in t0000 are quite special given that they many of them run
nested testcases to verify that testing functionality itself works as
expected. These nested testcases are realized by writing a new ad-hoc
test script which again sources test-lib.sh, where the new script is
created in a nested subdirectory located beneath the current trash
directory. We then execute the new test script with the nested
subdirectory as current working directory and explicitly re-export
TEST_OUTPUT_DIRECTORY to point to that directory.

While this works as expected in the general case, it falls apart when
the developer has TEST_OUTPUT_DIRECTORY explicitly defined either via
the environment or via config.mak and runs "make test". In that case,
test-lib.sh will clobber the value that we've just carefully set up to
instead contain what the developer has defined. As a result, the
TEST_OUTPUT_DIRECTORY continues to point at the root output directory,
not at the nested one.

This issue causes breakage in the 'test_atexit is run' test case: the
nested test case writes files into "../../", which is assumed to be the
parent's trash directory. But because TEST_OUTPUT_DIRECTORY already
points to to the root output directory, we instead end up writing those
files outside of the output directory. The parent test case will then
try to check whether those files still exist in its own trash directory,
which thus must fail now.

Fix the issue by adding a new TEST_OUTPUT_DIRECTORY_OVERRIDE variable.
If set, then we'll always override the TEST_OUTPUT_DIRECTORY with its
value after sourcing GIT-BUILD-OPTIONS.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>