Be more paranoid about quoting in pg_upgrade's test script.

Be more paranoid about quoting in pg_upgrade's test script.

Double-quote $PGDATA in "find" commands introduced by commit da9b580d8,
in case that path contains spaces or other special characters.

Adjust a few other places so that quoting is done more consistently.
None of the others are actual bugs AFAICS, but it's confusing to readers
if the same thing is done differently in different places.

Noted by Tels.

Discussion: https://postgr.es/m/c96303c04c360bbedaa04f90f515745b.squirrel@sm.webmail.pair.com