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 c737a2e5, so the lack of
field reports to date doesn't represent a lot of evidence that the problem
is rare.

contrib/pg_upgrade/test.sh

@@ -83,10 +83,10 @@ mkdir "$logdir"
 PGDATABASE="";        unset PGDATABASE
 PGUSER="";            unset PGUSER
 PGSERVICE="";         unset PGSERVICE
-PGSSLMODE=""          unset PGSSLMODE
+PGSSLMODE="";         unset PGSSLMODE
 PGREQUIRESSL="";      unset PGREQUIRESSL
 PGCONNECT_TIMEOUT=""; unset PGCONNECT_TIMEOUT
-PGHOST=""             unset PGHOST
+PGHOST="";            unset PGHOST
 PGHOSTADDR="";        unset PGHOSTADDR
 
 # Select a non-conflicting port number, similarly to pg_regress.c