test: DialectContext duplicates strawchemy.testing.MockContext #402

Once #399 is fixed, MockContext (src/strawchemy/testing/__init__.py) builds a mocked session whose bind reports the given dialect. DialectContext in tests/unit/utils.py does the same job, but builds its mocks with spec=.

Two options:

• replace DialectContext with MockContext in the unit tests
• give MockContext the stricter spec= mocks, then drop DialectContext

Either way, one implementation is left.