Fix bogus handling of $(X) (i.e., ".exe") in isolationtester Makefile.
I'm not sure why commit
1eb1dde049ccfffc42c80c2bcec14155c58bcc1f seems
to have made this start to fail on Cygwin when it never did before ---
but nonetheless, the coding was pretty bogus, and unlike the way we
handle $(X) anywhere else. Per buildfarm.