t7408: replace incorrect uses of test_must_fail
According to t/README, test_must_fail() should only be used to test for
failure in Git commands.
Replace the invocation of `test_must_fail test_path_is_file` with
`test_path_is_missing` since, in this test case, the path should not
exist at all.
In all the cases where `test_must_fail test_alternate_is_used` appears,
test_alternate_is_used() fails because test_line_count() cannot open the
non-existent $alternates_file. Replace
`test_must_fail test_alternate_is_used` with `test_path_is_missing` to
test for this directly.
Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>