Fix "no such column: id" output in tests

[simonw]

pytest
...
tests/test_custom_pages.py ........                                                                                                                                            [ 33%]
tests/test_database.py ......no such column: id
...                                                                                                                                               [ 35%]

[simonw]

$ pytest tests/test_database.py -v
================================================================================ test session starts =================================================================================
platform darwin -- Python 3.7.7, pytest-5.2.4, py-1.8.1, pluggy-0.13.1 -- /Users/simon/.local/share/virtualenvs/datasette-AWNrQs95/bin/python
cachedir: .pytest_cache
rootdir: /Users/simon/Dropbox/Development/datasette, inifile: pytest.ini
plugins: asyncio-0.10.0
collected 9 items                                                                                                                                                                    

tests/test_database.py::test_table_exists[tables0-True] PASSED                                                                                                                 [ 11%]
tests/test_database.py::test_table_exists[tables1-False] PASSED                                                                                                                [ 22%]
tests/test_database.py::test_get_all_foreign_keys PASSED                                                                                                                       [ 33%]
tests/test_database.py::test_table_names PASSED                                                                                                                                [ 44%]
tests/test_database.py::test_execute_write_block_true PASSED                                                                                                                   [ 55%]
tests/test_database.py::test_execute_write_block_false PASSED                                                                                                                  [ 66%]
tests/test_database.py::test_execute_write_fn_block_false no such column: id
PASSED                                                                                                               [ 77%]
tests/test_database.py::test_execute_write_fn_block_true PASSED                                                                                                                [ 88%]
tests/test_database.py::test_execute_write_fn_exception PASSED                                                                                                                 [100%]

================================================================================= 9 passed in 1.90s ==================================================================================