RE: [PATCH] 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. 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.