Fix unportable shell-script syntax in pg_upgrade's test.sh.
I discovered the hard way that on some old shells, the locution
FOO="" unset FOO
does not behave the same as
FOO=""; unset FOO
and in fact leaves FOO set to an empty string. test.sh was inconsistently
spelling it different ways on adjacent lines.
This got broken relatively recently, in commit
c737a2e56, so the lack of
field reports to date doesn't represent a lot of evidence that the problem
is rare.