Allow running tests with geltest

`geltest` [1] is `edb test` extracted out of Gel server codebase and
generalized to run any `unittest`-based suite.

Notable changes here:

1. `_testbase` moved to `_internal._testbase` and broken down along
   major case types (base, models, third-party ORM);

2. Test cases that involve model generation now reuse `std` reflection
   for better `mypy` cache utilization.

3. Test models are now reflected into (.gitignored) `tests/models` and
   relative imports in tests now work consistently (so instead of
   `from models.orm import Foo` write `from .models.orm import Foo` and
   have your editor LSP pick that up.  This obviates the need to run
   `tools/gen_models.py` (which I will remove in a subsequent commit).