chore: add test for SPA explore endpoints around missing datasets

[hughhhh]

SUMMARY

Added test to validate the proper response that we currently returning for missing/not found datasets

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #21303 (813d9d0) into master (8c2719b) will decrease coverage by 0.17%.
The diff coverage is n/a.

❗ Current head 813d9d0 differs from pull request most recent head 935fe24. Consider uploading reports for the commit 935fe24 to get more accurate results

@@            Coverage Diff             @@
##           master   #21303      +/-   ##
==========================================
- Coverage   66.61%   66.44%   -0.18%     
==========================================
  Files        1791     1791              
  Lines       68539    68539              
  Branches     7310     7310              
==========================================
- Hits        45656    45539     -117     
- Misses      21002    21119     +117     
  Partials     1881     1881              

Flag	Coverage Δ
hive	?
mysql	80.84% <ø> (+<0.01%)	⬆️
postgres	80.89% <ø> (+<0.01%)	⬆️
presto	?
python	81.00% <ø> (-0.36%)	⬇️
sqlite	79.44% <ø> (+<0.01%)	⬆️
unit	?

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

Impacted Files	Coverage Δ
superset/db_engines/hive.py	0.00% <0.00%> (-85.19%)	⬇️
superset/db_engine_specs/hive.py	71.09% <0.00%> (-16.02%)	⬇️
superset/db_engine_specs/presto.py	81.32% <0.00%> (-6.63%)	⬇️
superset/models/sql_lab.py	74.90% <0.00%> (-2.00%)	⬇️
superset/initialization/__init__.py	90.03% <0.00%> (-1.72%)	⬇️
superset/connectors/sqla/models.py	89.90% <0.00%> (-0.70%)	⬇️
superset/db_engine_specs/base.py	88.01% <0.00%> (-0.32%)	⬇️
superset/models/core.py	88.81% <0.00%> (-0.24%)	⬇️
superset/utils/core.py	89.88% <0.00%> (-0.12%)	⬇️
superset/explore/commands/get.py	91.95% <0.00%> (+2.29%)	⬆️

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[Antonio-RiveroMartnez]

Does this mean we are expecting a 200 when no dataset was found?

[AAfghahi]

Yeah same question.

[hughhhh]

yea this is the expected results whenever we hit explore with a unknown dataset, assuming product is okay with this for now but if this needs to change atleast we have a test to warrant expected result