permit `hdiutil` to exit with error code

Mysterious `hdiutil` errors are a longtime problem with the
test suite, and the largest cause of false Travis failure
reports.

What appears to happen is that `hdiutil` returns a valid
plist output, but also exits with an error code.

Recently, user errors have been reported which seem to be
related to the same problem (#4857).

After #4892 and #4887, we have have more assurances that mount errors
will be caught elsewhere, if the XML is incorrect/missing, or if the
mounts are not returned.

So, for this special case, it should be safe to simply
ignore the error code from `hdiutil`.