refactor(testing): remove FakeTime.start setter

[iuioiua]

See #5033 (comment)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.83%. Comparing base (635e062) to head (76223e1).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5050      +/-   ##
==========================================
- Coverage   92.20%   91.83%   -0.38%     
==========================================
  Files         487      487              
  Lines       38855    38854       -1     
  Branches     5443     5410      -33     
==========================================
- Hits        35827    35681     -146     
- Misses       2973     3116     +143     
- Partials       55       57       +2     

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[kt3k]

By this change, the error message changes from:

error: Uncaught (in promise) Error: cannot change start time after initialization

To:

error: Uncaught (in promise) TypeError: Cannot set property start of #<FakeTime> which has only 

I slightly prefer the 1st one because it explicitly says it's intentional by design. The latter sounds like the case is not considered by the author. WDYT?

[iuioiua]

I see having a method that is not useful as having poor design. I've added a note to counteract any possible confusion.

[kt3k]

LGTM