Fix portability issue with gzip in TAP test of pg_receivewal

Fix portability issue with gzip in TAP test of pg_receivewal

The OpenBSD implementation of gzip considers only files suffixed by "Z",
"gz", "z", "tgz" or "taz" as valid targets, discarding anything else and
making a command using --test exit with an error code of 512 if anything
invalid is found.  The test introduced in ffc9dda tested a WAL segment
suffixed as .gz.partial, enough to make the test fail.

Testing only a full segment is fine enough in terms of coverage, so
simplify the code by discarding the .gz.partial segment in this check.
This should be enough to make the test pass with OpenBSD environments.

Per report from curculio.

Discussion: https://postgr.es/m/YPAdf9r5aJbDoHoq@paquier.xyz