Move the test for Win32's FindExt.pm to t/porting, and run it automatically.

Move the test for Win32's FindExt.pm to t/porting, and run it automatically.

The test was written as part of the work on migrating modules to cpan/ and
dist/, but at that time at least one of FindExt.pm and Configure was buggy with
the classification of at least one module (XS or non-XS). As that is now fixed,
move the test to t/porting, and run it routinely. This will ensure that the
Win32 code's classification of modules will remain consistent with Configure's.