tests/prediction-unicode.test: env -u is not POSIX

Seen to fail on OpenBSD 5.2.

src/tests/prediction-unicode.test

@@ -72,10 +72,14 @@ baseline()
 post()
 {
     # Look for bad output: ')' or \374
-    if [ -n "$(env -u LC_ALL -u LC_CTYPE -u LANGUAGE LANG=C egrep "%output %0 (\)|$(printf \\374))" "$(basename "$0").d/baseline.tmux.log")" ]; then
+    (
-        exit 1
+	unset LC_ALL
-    fi
+	unset LC_CTYPE
-    exit 0
+	unset LANGUAGE
+	! env LANG=C egrep -q "%output %0 (\)|$(printf \\374))" "$(basename "$0").d/baseline.tmux.log"
+	# Implicit exitcode return.
+    )
+    return $?
 }
 
 case $1 in