Add container reset feature + postgres impl

[orlangure]

This feature allows to reset containers to their initial state between tests, when reused.

As the first example, I implemented Reset function for postgres preset. It drops schemas and deletes users, and then calls the regular init function to restore the state requested by user options in the beginning. New postgres test suite demonstrates how the feature can be used.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 58.33333% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.41%. Comparing base (f6a5b27) to head (4ab136b).
⚠️ Report is 93 commits behind head on master.

Files with missing lines	Patch %	Lines
preset/postgres/preset.go	51.21%	10 Missing and 10 partials ⚠️

❌ Your patch check has failed because the patch coverage (58.33%) is below the target coverage (70.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #699      +/-   ##
==========================================
- Coverage   85.80%   85.41%   -0.39%     
==========================================
  Files          49       49              
  Lines        2310     2358      +48     
==========================================
+ Hits         1982     2014      +32     
- Misses        170      178       +8     
- Partials      158      166       +8     

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[cgeers]

very useful. this looks great! 👍