[web] Use toBeDefined in web Olm tests

Summary: In D7090, @bartek left some feedback that I accidentally only addressed in the keyserver tests. This is a small follow-up to address that feedback in the web tests as well.

Test Plan: Run tests

Reviewers: bartek, atul

Reviewed By: atul

Subscribers: tomek, bartek

Differential Revision: https://phab.comm.dev/D7372