Add nested lazy "show source" test

Summary:
In main Hermes, this test fails because of the multi-RuntimeModule
setup, so we're not able to locate the function source table that
actually contains the source.

Add a test to make sure this never regresses in SH.

Reviewed By: tmikov

Differential Revision: D67317073

fbshipit-source-id: 981d241eb9ad48f308287d18cfab0bab5feda127

test/hermes/lazy-function-toString.js

@@ -13,6 +13,9 @@
 function foo(x) { "show source"; }
 
 (function bar() {
+  function baz() { "show source"; return 'baz;' }
     print(foo.toString());
     // CHECK: function foo(x) { "show source"; }
+    print(baz.toString());
+    // CHECK: function baz() { "show source"; return 'baz;' }
 })()