Don't skip popen.test on MinGW

[Eli Zaretskii]

Another issue related to open-process is that popen.test was being
skipped because if-supported returns #f.  I fixed it thusly:

     (define-syntax-rule (if-supported body ...)
       (if (or (provided? 'fork)
               ((string-contains %host-type "-mingw32")))
           (begin body ...)))

Is this OK?  Again, perhaps the test should be removed, and popen.test
should be run unconditionally?