Fix TMPDIR noexec check to explicitly exit with a zero exit status on success

The TMPDIR check implemented in a4556a73 incorrectly reports that
TMPDIR cannot hold executables on 4.3.11(1) on Ubuntu 14.04.1 LTS.
This is because a script containing no commands returns a non-zero
exit code.  Contrast the following:

  bash 3.2.53(1) on OS X 10.9.5:

    $ bash -c '' && echo SUCCESS
    SUCCESS

  bash 4.3.11(1) on Ubuntu 14.04.1 LTS:

    $ bash -c '' || echo FAIL
    FAIL

This patch modifies the test script to explicitly call `exit 0` to
ensure that a successful exit code is returned if the script executes
successfully.