t/README: test_must_fail is for testing Git

t/README: test_must_fail is for testing Git

When a test wants to make sure there is no <string> in an output
file, we should just say "! grep string output".

"test_must_fail" is there only to test Git command and catch unusual
deaths we know about (e.g. segv) as an error, not as an expected
failure.  "test_must_fail grep string output" is unnecessary, as
we are not making sure the system binaries do not dump core or
anything like that.

Signed-off-by: Junio C Hamano <gitster@pobox.com>