Add tests to ensure resolved modules are singleton instances

[jryans]

There may be cases today where require('<X>') gives different instances depending on where you are requiring from. This is generally not expected or intended for a well-functioning JS module system. (It is believed this may occur today across the high-level boundaries like Riot vs. SDKs, perhaps because we have not yet integrated them into a single build.)

We should ensure this works as expected for a consistent developer experience and add tests to ensure it stays that way going forward.

See also: matrix-org/matrix-react-sdk#2770 (comment)

[Johennes]

Closing as this doesn't appear to have caused any practical issues since March 2019.