[3.11] gh-108558: Improve sqlite3 row factory tests (GH-108578) (#108616)

Add test_sqlite_row_keys() to explicitly test sqlite3.Row.keys().

Cleanups:
- Reduce test noise by converting docstrings to regular comments
- Reduce boilerplate code by adding a setUp() method to RowFactoryTests

(cherry picked from commit 6eaddc10e972273c1aed8b88c538e65e4773496e)

Co-authored-by: Edward Schauman-Haigh <142528725+EddInSverige@users.noreply.github.com>
Co-authored-by: Erlend E. Aasland <erlend@python.org>