Add test for dovecot unsolicited OK progress message.

[mordak]

Follow up on #179.

We seem to have handled this with the recent work on UnsolicitedResponse, so I just added a test to make sure we continue to handle this.

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This change is 

[codecov]

Codecov Report

Merging #196 (e79c0ae) into master (18ebf56) will increase coverage by 0.26%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #196      +/-   ##
==========================================
+ Coverage   77.75%   78.02%   +0.26%     
==========================================
  Files          15       15              
  Lines        1722     1734      +12     
==========================================
+ Hits         1339     1353      +14     
+ Misses        383      381       -2     

Impacted Files	Coverage Δ
src/parse.rs	88.10% <100.00%> (+0.41%)	⬆️
src/types/unsolicited_response.rs	40.00% <0.00%> (+5.71%)	⬆️

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[jonhoo]

Good call, thanks!