[WIP] Switch python tests to testgres

[ololobus]

I have added TCP-DS in a separate PR https://github.com/postgrespro/pg_query_state/pull/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%)	:arrow_up:
pg_query_state.c	84.48% <0%> (+10.57%)	:arrow_up:

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