libregex: Mark gnuext test as an expected fail

The test was added prematurely as a goal to reach with the GNU extension
functionality, but the functionality has not yet been introduced. Mark it as
an expected fail until that point.

lib/libregex/tests/libregex_test.sh

@@ -29,6 +29,10 @@ check()
 {
 	local dataname="${1}"; shift
 
+	if [ "${dataname}" == "gnuext" ]; then
+		atf_expect_fail "GNU extensions are not currently implemented"
+	fi
+
 	prog="$(atf_get_srcdir)/h_regex"
 	data="$(atf_get_srcdir)/data/${dataname}.in"