tests: avoid a spurious failure with Solaris /bin/sh

tests: avoid a spurious failure with Solaris /bin/sh

The /bin/sh shell on Solaris is dumb enough not to set the exit
status to 127 after the execution of a non-existing command is
attempted:

  $ /bin/sh -c 'nonesuch'; echo stat = $?
  /bin/sh: nonesuch: not found
  stat = 1

This means that the missing script, when run through that shell,
cannot discriminate between a real failure of a maintainer tool
and a failure due to its absence.  This is not a big deal in
practice (especially because all the 'missing' invocations in
our Makefiles are done with $(SHELL), and that is almost surely
set by configure to a proper POSIX shell), but was causing an
annoying failure in our testsuite.  Fix it.

* t/missing3.sh: If 'missing' is run with a /bin/sh shell suffering
from the just-described bug, skip the check that would spuriously
fail due to that bug.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>