Avoid unportable shell syntax in pg_upgrade's test script.
Most of test.sh uses traditional backtick syntax for command substitution,
but commit
da9b580d8 introduced two uses of $(...) syntax, which is not
recognized by very old shells. Bring those into line with the rest.
Victor Wagner
Discussion: https://postgr.es/m/
20180720153820.
69e9ae6c@fafnir.local.vm