Require output fixture extension to match sourceType output.

packages/babel-plugin-transform-runtime/test/fixtures/runtime/custom-runtime/output.mjs

@@ -0,0 +1,24 @@
+import _regeneratorRuntime from "foo/regenerator";
+import _Symbol from "foo/core-js/symbol";
+
+var _marked =
+/*#__PURE__*/
+_regeneratorRuntime.mark(giveWord);
+
+import foo, * as bar from "someModule";
+export const myWord = _Symbol("abc");
+export function giveWord() {
+  return _regeneratorRuntime.wrap(function giveWord$(_context) {
+    while (1) switch (_context.prev = _context.next) {
+      case 0:
+        _context.next = 2;
+        return myWord;
+
+      case 2:
+      case "end":
+        return _context.stop();
+    }
+  }, _marked, this);
+}
+foo;
+bar;