Add file existence test for Porting/makerel

Add file existence test for Porting/makerel

This came up in the release of 5.39.7. Previously autodie.pm had been
upgraded, which removed the file touch_me from the Perl distribution.
The name touch_me was still mentioned in Porting/makerel , but this
only became apparent when makerel was used to actually build a tarball.

This test crudely parses the list of filenames from Porting/makerel
and checks that all the files referenced there exist.