Non-portable shell code in pg_upgrade tap tests

Collegues,

I've discovered that in the branch REL_11_STABLE there is shell script
src/bin/pg_upgrade/test.sh which doesn't work under Solaris 10.
(it uses $(command) syntax with is not compatible with original
Solaris /bin/sh)

I was quite surprised that this problem goes unnoticed on big buildfarm,
but after some investigation found out that both Solaris machines in
that buildfarm don't configure postgres with --enable-tap-tests.

Offending code is:

# make sure all directories and files have group permissions, on Unix hosts
# Windows hosts don't support Unix-y permissions.
case $testhost in
MINGW*) ;;
*) if [ $(find ${PGDATA} -type f ! -perm 640 | wc -l) -ne 0 ]; then
echo "files in PGDATA with permission != 640";
exit 1;
fi ;;
esac

case $testhost in
MINGW*) ;;
*) if [ $(find ${PGDATA} -type d ! -perm 750 | wc -l) -ne 0 ]; then
echo "directories in PGDATA with permission != 750";
exit 1;
fi ;;
esac

It is quite easy to replace $() syntax with backticks. Backticks are
not nestable and considered unsafe by modern shell scripting style
guides, but they do work with older shells.

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