tests: introduce test_must_fail

When we expect a git command to notice and signal errors, we
carelessly wrote in our tests:

    test_expect_success 'reject bogus request' '
        do something &&
        do something else &&
        ! git command
    '

but a non-zero exit could come from the "git command" segfaulting.

A new helper function "tset_must_fail" is introduced and it is
meant to be used to make sure the command gracefully fails (iow,
dying and exiting with non zero status is counted as a failure
to "gracefully fail").  The above example should be written as:

    test_expect_success 'reject bogus request' '
        do something &&
        do something else &&
        test_must_fail git command
    '

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

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