Fix potential double fclose in test_extras (cordtest)

Fix potential double fclose in test_extras (cordtest)

* cord/tests/cordtest.c (test_extras): Explicitly clear cord handles
obtained from CORD_from_file(), invoke GC_gcollect and
GC_invoke_finalizers to force f1a, f1b, f2 handles to be closed before
removing the files; remove the second attempts to remove the files on
rename failure (print a warning instead) as it might cause double
fclose (or double free); add comments.