gh-108558: Improve sqlite3 row factory tests (#108578)

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

Co-authored-by: Erlend E. Aasland <erlend@python.org>