Fix spinedb_api ResourceWarnings in tests

[ptsavol]

Dispose lingering engines. Fixes a lot of ResourceWarnings.

Related to spine-tools/Spine-Toolbox#3336

Checklist before merging

• Unit tests have been added/updated accordingly
• Code has been formatted by black & isort
• Unit tests pass

[ptsavol]

Added a couple of engine.dispose()'s. The undisposed engines bleed ResourceWarnings into spine_items, toolbox, and engine tests.

[soininen]

I had a few questions about this, please see the comments.

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.15%. Comparing base (303a427) to head (38c562c).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
spinedb_api/db_mapping.py	60.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #625   +/-   ##
=======================================
  Coverage   86.15%   86.15%           
=======================================
  Files          83       83           
  Lines       11475    11477    +2     
  Branches     1636     1636           
=======================================
+ Hits         9886     9888    +2     
  Misses       1238     1238           
  Partials      351      351           

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