Fix issues with TAP tests of pg_verify_checksums

Fix issues with TAP tests of pg_verify_checksums

Two issues have been spotted and get fixed here:
- When checking for corrupted files, make sure that pg_verify_checksums
complains about the correct file.  In order to make the logic more
robust, all files created are immediately removed once checks on them
are done.  The error message generated by pg_verify_checksums also now
includes the file name it sees as corrupted.
- Before running corruption-related tests, empty files are generated
which used names mapping with the corrupted files, potentially leading
to conflicts.  So use different set of names for both.

Author: Michael Banck
Discussion: https://postgr.es/m/20181119181119.GC23740@nighthawk.caipicrew.dd-dns.de