imap-proto: make quoted tests more precise

[djc]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.29%. Comparing base (502161c) to head (142eb7d).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #180      +/-   ##
==========================================
+ Coverage   78.26%   78.29%   +0.02%     
==========================================
  Files          22       22              
  Lines        3396     3400       +4     
==========================================
+ Hits         2658     2662       +4     
  Misses        738      738              

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