chore: update fixtures scope that are loading data into the analytical db

[bkyryliuk]

SUMMARY

Play with fixture scopes to see if tests will be faster
Hive and presto DBs are slow, we should load test data only once rather than do it for every module / test function

Speed improvements:
Python Presto/Hive / test-postgres-presto (3.8) 38m vs 92 m
Python Presto/Hive / test-postgres-hive 39m vs 53 m

There are probably other tests as well that can benefit from a similar improvement

[codecov]

Codecov Report

Merging #17780 (c69af00) into master (bcd698d) will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #17780      +/-   ##
==========================================
+ Coverage   67.72%   67.79%   +0.07%     
==========================================
  Files        1605     1605              
  Lines       64205    64207       +2     
  Branches     6790     6790              
==========================================
+ Hits        43481    43529      +48     
+ Misses      18868    18822      -46     
  Partials     1856     1856              

Flag	Coverage Δ
hive	81.80% <ø> (+<0.01%)	⬆️
mysql	82.18% <ø> (+<0.01%)	⬆️
postgres	82.23% <ø> (+<0.01%)	⬆️
presto	82.10% <ø> (?)
python	82.72% <ø> (+0.15%)	⬆️
sqlite	81.91% <ø> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
superset/dashboards/dao.py	95.55% <0.00%> (-0.75%)	⬇️
superset/dashboards/commands/update.py	84.28% <0.00%> (+0.46%)	⬆️
superset/models/core.py	90.00% <0.00%> (+0.73%)	⬆️
superset/sql_lab.py	81.64% <0.00%> (+0.78%)	⬆️
superset/extensions.py	93.90% <0.00%> (+1.21%)	⬆️
superset/connectors/sqla/models.py	88.42% <0.00%> (+1.34%)	⬆️
superset/db_engine_specs/presto.py	90.39% <0.00%> (+6.05%)	⬆️

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[betodealmeida]

This is great!

I've been working on converting integration tests to unit tests, hopefully we can keep the number of integration tests down to a bare minimum in the near future.