bug(web): automated test instability for prediction test

[jahorton]

          Build failure seen for ["Test: Language Modeling Layer (Common)"](https://build.palaso.org/buildConfiguration/Keyman_Common_LMLayer_TestPullRequests/473674):

15:56:57 
  LMLayer using the trie model

15:56:57 
    Prediction

15:56:57 
    Regression tests

15:56:57 
      should use the default searchTermToKey()

15:56:58 
        expected +0 to be at least 1

15:56:58 
        AssertionError: expected +0 to be at least 1
            at file:///C:/BuildAgent/work/99b311828f4ee7c/keyman/common/predictive-text/unit_tests/headless/worker-trie-integration.js:85:16

The worker's "test mode" is on, so we shouldn't be exiting early... but perhaps recent changes from #11784 and its predecessors have had an unintended consequence here.

Originally posted by @jahorton in #11264 (comment)