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

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>