Use a rollback strategy with single engine in tests

[GianlucaFicarelli]

Use a single engine in tests to avoid truncating tables after each test.

Local execution:

make destroy
time make test-local

- commit 178098e5 (main):
1081 passed, 29 warnings in 266.58s (0:04:26) 
user=137.55s system=8.08s cpu=50% total=4:46.69

- commit bf31c6b4 (PR)
1081 passed, 29 warnings in 168.89s (0:02:48)
user=133.22s system=6.28s cpu=74% total=3:07.74

CI execution:

- commit 178098e5 (main):
1081 passed, 29 warnings in 532.17s (0:08:52)
https://github.com/openbraininstitute/entitycore/actions/runs/30009134111/job/89212447652

- commit 84bfe2c7 (PR, intermediate commit)
1081 passed, 29 warnings in 403.45s (0:06:43)
https://github.com/openbraininstitute/entitycore/actions/runs/30110409413/job/89538236571

- commit b066a5e8 (PR, last commit)
1083 passed, 29 warnings in 425.86s (0:07:05)
https://github.com/openbraininstitute/entitycore/actions/runs/30269625501/job/89988649102?pr=683

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
pytest	97.84% <100.00%> (-0.02%)	⬇️

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

Files with missing lines	Coverage Δ
app/application.py	96.15% <100.00%> (+0.07%)	⬆️
app/db/model.py	99.15% <ø> (ø)
app/db/session.py	78.04% <100.00%> (+1.73%)	⬆️
app/dependencies/db.py	86.66% <100.00%> (-13.34%)	⬇️
app/queries/common.py	98.24% <100.00%> (+0.01%)	⬆️

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[eleftherioszisis]

nice!