[WIP] Switch python tests to testgres

[ololobus]

I have added TCP-DS in a separate PR #17, since it is difficult to do two huge things at once (migrating to testgres and adding TCP-DS). Although I still think that tesgress is a powerful Postgres testing framework and hope that migration to it will help us to unify our testing routines across different projects. So I keep this PR and mark it as work-in-progress for now.

[codecov-io]

Codecov Report

Merging #16 into master will increase coverage by 9.16%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #16      +/-   ##
==========================================
+ Coverage   77.09%   86.25%   +9.16%     
==========================================
  Files           4        2       -2     
  Lines         502      502              
==========================================
+ Hits          387      433      +46     
+ Misses        115       69      -46

Impacted Files	Coverage Δ
pg_bin/include/postgresql/server/utils/palloc.h
pg_bin/include/postgresql/server/storage/s_lock.h
signal_handler.c	95.18% <0%> (+3.71%)	⬆️
pg_query_state.c	84.48% <0%> (+10.57%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0ebe691...7e81aaf. Read the comment docs.