fix: Add deprecation warning when importing legacy testing helpers #3752

Job	Run time
External Tests	2m 11s
Test on 3.7 (tests) / ubuntu-latest / SQLAlchemy: 2.*	1m 17s
Test on 3.8 (tests) / ubuntu-latest / SQLAlchemy: 2.*	1m 8s
Test on 3.9 (tests) / ubuntu-latest / SQLAlchemy: 2.*	1m 10s
Test on 3.10 (tests) / ubuntu-latest / SQLAlchemy: 2.*	1m 6s
Test on 3.11 (tests) / ubuntu-latest / SQLAlchemy: 2.*	1m 32s
Test on 3.7 (tests) / macos-latest / SQLAlchemy: 2.*	2m 26s
Test on 3.8 (tests) / macos-latest / SQLAlchemy: 2.*	2m 24s
Test on 3.9 (tests) / macos-latest / SQLAlchemy: 2.*	2m 7s
Test on 3.10 (tests) / macos-latest / SQLAlchemy: 2.*	2m 12s
Test on 3.11 (tests) / macos-latest / SQLAlchemy: 2.*	2m 46s
Test on 3.7 (tests) / windows-latest / SQLAlchemy: 2.*	2m 54s
Test on 3.8 (tests) / windows-latest / SQLAlchemy: 2.*	2m 54s
Test on 3.9 (tests) / windows-latest / SQLAlchemy: 2.*	3m 3s
Test on 3.10 (tests) / windows-latest / SQLAlchemy: 2.*	2m 37s
Test on 3.11 (tests) / windows-latest / SQLAlchemy: 2.*	2m 49s
Test on 3.11 (tests) / ubuntu-latest / SQLAlchemy: 1.*	1m 9s
Test on 3.10 (doctest) / ubuntu-latest / SQLAlchemy: 2.*	40s
Test on 3.8 (mypy) / ubuntu-latest / SQLAlchemy: 2.*	1m 3s
Coverage	50s
	38m 18s

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