t3407: run tests in $TEST_DIRECTORY

t3407: run tests in $TEST_DIRECTORY

Commit 97b88dd58c ("git-rebase.sh: Fix --merge --abort failures when
path contains whitespace", 2008-05-04) started running these tests in
a subdirectory with a space in its name. At that time $TEST_DIRECTORY
did not contain a space but shortly after in 4a7aaccd83 ("Rename the
test trash directory to contain spaces.", 2008-05-04) $TEST_DIRECTORY
was changed to contain a space so we no longer need to run these tests
in a subdirectory.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>