Skip JS backtraces test on --native

* It would need the polyglot image to be built, which is usually not the
  case in CI.

Author: eregon

test/truffle/integration/backtraces.sh

@@ -5,9 +5,11 @@ source test/truffle/common.sh.inc
 for f in test/truffle/integration/backtraces/*.rb
 do
   echo "$f"
-  options=""
-  if [ "$(basename "$f")" = "javascript.rb" ]; then
-    options="--polyglot --experimental-options --single_threaded"
+  if [ "$(basename "$f")" != "javascript.rb" ]; then
+    jt ruby --no-core-load-path "$f"
   fi
-  jt ruby --no-core-load-path $options "$f"
 done
+
+if [ "$(jt ruby -e 'p TruffleRuby.native?')" = "false" ]; then
+  jt ruby --no-core-load-path --polyglot --experimental-options --single_threaded test/truffle/integration/backtraces/javascript.rb
+fi