Test for change to cache templates by site, not contents (#972)

* Test for change to cache templates by site, not contents

These tests are against a specification change based on discussion in
https://github.com/tc39/ecma262/issues/840

The tests here passed on SpiderMonkey but failed on other
implementations, which implement the current specification.

* Add a test that caching is by source location, not function identity

* Update existing tests to reference the spec properly