Bring back skip_without_user_xattrs helper function

We'll need this to be able to skip system-mode tests without xattrs.
This partially reverts commit a53a752a35.

Signed-off-by: Simon McVittie <smcv@collabora.com>

tests/libtest.sh

@@ -309,6 +309,14 @@ run_sh () {
     ${CMD_PREFIX} flatpak run --command=bash ${ARGS-} org.test.Hello -c "$*"
 }
 
+skip_without_user_xattrs () {
+    touch ${TEST_DATA_DIR}/test-xattrs
+    if ! setfattr -n user.testvalue -v somevalue ${TEST_DATA_DIR}/test-xattrs; then
+        echo "1..0 # SKIP this test requires xattr support"
+        exit 0
+    fi
+}
+
 skip_without_bwrap () {
     if [ -z "${FLATPAK_BWRAP:-}" ]; then
         # running installed-tests: assume we know what we're doing