test: /postgres engine spec test coverage

[kstrz]

SUMMARY

Increase postgres engine spec test coverage.
Fix estimate_statement_cost for invalid syntax

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TEST PLAN

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Requires DB Migration.
• Confirm DB Migration upgrade and downgrade tested.
• Introduces new feature or API
• Removes existing feature or API

[villebro]

A few quick first pass comments

[kstrz]

Hey! Could you look at it? @willbarrett @dpgaspar

[codecov-io]

Codecov Report

Merging #12490 (426c502) into master (7aba4c2) will decrease coverage by 2.45%.
The diff coverage is 81.81%.

@@            Coverage Diff             @@
##           master   #12490      +/-   ##
==========================================
- Coverage   66.29%   63.84%   -2.46%     
==========================================
  Files        1015      486     -529     
  Lines       49554    29938   -19616     
  Branches     5079        0    -5079     
==========================================
- Hits        32854    19114   -13740     
+ Misses      16562    10824    -5738     
+ Partials      138        0     -138     

Flag	Coverage Δ
cypress	?
javascript	?
python	63.84% <81.81%> (-0.25%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset/databases/schemas.py	99.45% <ø> (-0.55%)	⬇️
superset/db_engine_specs/druid.py	88.63% <33.33%> (+27.27%)	⬆️
superset/db_engine_specs/postgres.py	96.05% <89.47%> (+10.08%)	⬆️
superset/sql_validators/postgres.py	50.00% <0.00%> (-50.00%)	⬇️
superset/db_engine_specs/mysql.py	79.59% <0.00%> (-12.25%)	⬇️
superset/db_engine_specs/sqlite.py	65.62% <0.00%> (-9.38%)	⬇️
superset/databases/commands/create.py	83.67% <0.00%> (-8.17%)	⬇️
superset/databases/commands/update.py	85.71% <0.00%> (-8.17%)	⬇️
superset/databases/commands/test_connection.py	84.78% <0.00%> (-4.35%)	⬇️
superset/utils/celery.py	96.42% <0.00%> (-3.58%)	⬇️
... and 543 more

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[dpgaspar]

nit: can you post the current coverage and the new coverage after the PR?

[villebro]

@kstrz this needs to be rebased

[kstrz]

Coverage for superset/db_engine_specs/postgres.py on master: 85.96% , on current branch: 96.05%