t/t3700: convert two uses of negation operator '!' to use test_must_fail

These two lines use the negation '!' operator to negate the result of a simple command. Since these commands do not contain any pipes or other complexities, the test_must_fail function can be used and is preferred since it will additionally detect termination due to a signal. This was noticed because the second use of '!' does not include a space between the '!' and the opening parens. Ksh interprets this as follows: !(pattern-list) Matches anything except one of the given patterns. Ksh performs a file glob using the pattern-list and then tries to execute the first file in the list. If a space is added between the '!' and the open parens, then Ksh will not interpret it as a pattern list, but in this case, it is preferred to use test_must_fail, so lets do so. Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil> Signed-off-by: Junio C Hamano <gitster@pobox.com>