Modifying the permissions of a persistent file isn't really much nicer
than modifying its contents, even if git doesn't currently notice it.
Adjust the test script to make a copy and set the permissions of that
instead.

Michael Paquier, per a gripe from me.  Back-patch to 9.5 where these
tests were introduced.

Discussion: https://postgr.es/m/14836.1494885946@sss.pgh.pa.us