Revert "Rename all test files from JS -> ES6"

This reverts commit 2abe85b834.

test/javascripts/models/session-test.js.es6

@@ -0,0 +1,12 @@
+import Session from "discourse/models/session";
+
+QUnit.module("model:session");
+
+QUnit.test("highestSeenByTopic", assert => {
+  const session = Session.current();
+  assert.deepEqual(
+    session.get("highestSeenByTopic"),
+    {},
+    "by default it returns an empty object"
+  );
+});