ble-pybricks-service/sagas: simplify test

[dlech]

I meant to change this before merging the previous commit, but missed it.

The mismatch test was hard to understand and didn't increase coverage, so let's leave it out.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.61%. Comparing base (baaa9a7) to head (75e55b8).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2321      +/-   ##
==========================================
+ Coverage   72.47%   72.61%   +0.13%     
==========================================
  Files         201      201              
  Lines        5127     5163      +36     
  Branches     1086     1094       +8     
==========================================
+ Hits         3716     3749      +33     
- Misses       1374     1377       +3     
  Partials       37       37              

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