Unit tests for Olm's new X3DH methods

Summary:
This adds a quick test test to confirm Anunay's new methods work on web and keyserver that are introduced in the [fork](D7089).

Note that more extensive testing was done by updating the upstream tests in the fork, which include a Bob/Alice scenario in JavaScript.

Depends on D7091

Test Plan: Run the tests

Reviewers: anunay, jon, varun, bartek

Reviewed By: bartek

Subscribers: bartek, tomek, atul

Differential Revision: https://phab.comm.dev/D7090