t3400: make test clean up after itself

t3400: make test clean up after itself

This test intentionally creates a file which causes rebase to fail, thus
it is important that this file be deleted before subsequent tests are
run which are not expecting such a failure. In the past, the common way
to ensure cleanup (regardless of whether the test succeeded or failed)
was either for the next test to perform the previous test's cleanup as
its first step or to do the cleanup at global scope outside of any
tests. With the introduction of 'test_when_finished', however, tests can
be responsible for their own cleanup. Therefore, update this test to
clean up after itself.

A bit of history: This 'rm' invocation was moved from within the body of
the following test to global scope by bffd750adf (rebase: improve error
message when upstream argument is missing, 2010-05-31), which postdates,
by about a month, introduction of 'test_when_finished' in 3bf7886705
(test-lib: Let tests specify commands to be run at end of test,
2010-05-02).

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>