[Bug testsuite/65191] keep interesting core files and remove others

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65191

--- Comment #1 from vries at gcc dot gnu.org ---
Created attachment 34854
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=34854&action=edit
Demonstrator patch

Run with f.i. gfortran.dg/block_6.f08 to see that the core is cleaned up.