pgsql: Fix unportable shell-script syntax in pg_upgrade's test.sh.

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.

Branch
------
REL9_2_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/d24d6691930e5f3a95e9daece2f299a076c80c15

Modified Files
--------------
contrib/pg_upgrade/test.sh |    4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)