Create a centralised pytest fixture to provide controllable stdin and
captured stdout in all tests. Simplify DummyIO/DummyIn and remove the
custom DummyOut implementation and make use of pytest builtin fixtures.
Create a centralised pytest fixture to provide controllable stdin and
captured stdout that can be applied to any tests, regardless whether
they are based on pytest or unittest.
* `io` fixture can be used as a fixture in pytest-based tests
* `IOMixin` can be used to attach `io` attribute to any test class,
including `unittest.TestCase`
Moved tests related to ui into own folder.
Moved 'modify' command tests into own file.
Moved 'write' command tests into own file.
Moved 'fields' command tests into own file.
Moved 'do_query' test into own file.
Moved 'list' command tests into own file.
Moved 'remove' command tests into own file.
Moved 'move' command tests into own file.
Moved 'update' command tests into own file.
Moved 'show_change' test into test_import file.
Moved 'summarize_items' test into test_import file.
Moved 'completion' command test into own file.
