Add test cases for pg_dump output parsing

[RauliL]

Introduce test cases for the PostgreSQL database backend.

[codecov]

Codecov Report

Merging #3 into master will increase coverage by 8.74%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master       #3      +/-   ##
==========================================
+ Coverage   81.91%   90.66%   +8.74%     
==========================================
  Files           8        8              
  Lines         376      407      +31     
==========================================
+ Hits          308      369      +61     
+ Misses         68       38      -30

Impacted Files	Coverage Δ
database_sanitizer/tests/test_dump_postgres.py	100% <100%> (ø)	⬆️
database_sanitizer/dump/postgres.py	100% <0%> (+75%)	⬆️

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