tests: Example of retry() -> retry_until()

[tony]

@categulario If you'd like to start moving them over, let me know, you can create a new PR doing it

if not i will Friday night or over the weekend

cc: tmux-python/libtmux#372, #620, #704

[codecov]

Codecov Report

Merging #776 (c768be0) into master (c768be0) will not change coverage.
The diff coverage is n/a.

❗ Current head c768be0 differs from pull request most recent head 31ea6c3. Consider uploading reports for the commit 31ea6c3 to get more accurate results

@@           Coverage Diff           @@
##           master     #776   +/-   ##
=======================================
  Coverage   75.59%   75.59%           
=======================================
  Files          16       16           
  Lines        1254     1254           
  Branches      322      322           
=======================================
  Hits          948      948           
  Misses        220      220           
  Partials       86       86           

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[tony]

#777