bug(web): LMLayer unit test instability

**Describe the bug**
There seems to be some instability in one or two of our lm-layer unit tests:

https://build.palaso.org/viewLog.html?buildId=268424&buildTypeId=Keyman_Common_LMLayer_TestPullRequests

It's presently unclear if this is something that only shows in BrowserStack. I _feel_ like that's not the case, but only due to a vague memory from issues I've seen during development in the past.