feat: deprecate /superset/testconn and migrate to api v1

[zephyring]

SUMMARY

Following plan to gradually deprecate "REST" endpoints on /superset namespace.

Deprecates /superset/testconn. Proposed removal on Superset 3.0
New endpoint /api/v1/database/test_connection

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 #20002 (9ca119d) into master (62e1c34) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master   #20002   +/-   ##
=======================================
  Coverage   66.28%   66.28%           
=======================================
  Files        1712     1712           
  Lines       63968    63969    +1     
  Branches     6731     6731           
=======================================
+ Hits        42404    42405    +1     
  Misses      19853    19853           
  Partials     1711     1711           

Flag	Coverage Δ
hive	53.65% <0.00%> (-0.01%)	⬇️
mysql	81.99% <100.00%> (+<0.01%)	⬆️
postgres	82.04% <100.00%> (+<0.01%)	⬆️
presto	53.51% <0.00%> (-0.01%)	⬇️
python	82.47% <100.00%> (+<0.01%)	⬆️
sqlite	81.78% <100.00%> (+<0.01%)	⬆️
unit	48.63% <0.00%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
superset/views/core.py	77.38% <100.00%> (+0.01%)	⬆️

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