Make pg_regress clean out the testtablespace directory only on Windows.

Make pg_regress clean out the testtablespace directory only on Windows.
On other platforms it's better to let the Makefile handle it, but we want
the regression tests to be invokable without make on Windows.  A batch
file would be a better solution, but no time for that before 8.3.
Per my discovery that this breaks testing under SELinux, and subsequent
discussion.