add test isolation manager

[davidism]

Add a test_isolation context manager method (and async variant). This turns the code previously described in the "Avoid Writing Data" section of the testing docs into a context manager. The code was pretty complex, this hides that in a simple with block for users. Now the code is much shorter:

@pytest.fixture
def app(monkeypatch):
    app = create_app({
        "SQLALCHEMY_ENGINES": {"default": "postgresql:///project-test"}
    })

    with db.test_isolation():
        yield app

This also works with unittest:

class TestDatabase(unittest.TestCase):
    def setUp(self):
        self.enterContext(db.test_isolation())

Any changes to the database within the with block will be rolled back when it exits.

Our tests use SQLite, and we call out in the docs that you don't need this isolation code and can just blow away the in-memory database for each test in that case. However, it does work with SQLite, but SQLite's transaction handling needs to be fixed: https://docs.sqlalchemy.org/en/20/dialects/sqlite.html#sqlite-transactions. So our tests serve as a demonstration for how to use this with SQLite.