tests: set TEST_OUTPUT_DIRECTORY to the CGit test directory

tests: set TEST_OUTPUT_DIRECTORY to the CGit test directory

By default, Git's test suite puts the trash directories and test-results
directory into its own directory, not that containing the tests being
run.  This is less convenient for inspecting test failures, so set the
output directory to CGit's tests/ directory instead.

Note that there is currently a bug in Git whereby it will create the
trash directories in our tests/ directory regardless of the value of
TEST_OUTPUT_DIRECTORY, and then fail to remove them once the tests are
done.  This change does currently affect the location of the
test-results/ directory though.

Signed-off-by: John Keeping <john@keeping.me.uk>