Gather pytest files in a subdirectory

[rolandwalker]

Description

Gather pytest files in a subdirectory separated from behave test files.

Incidentally delete test.txt (replacing with an echo command) and delete a very old file test_plan.wiki.

Motivation: clarify the layout to help the AI write and run tests.

Checklist

• I added this contribution to the changelog.md file.
• I added my name to the AUTHORS file (or it's already there).
• To lint and format the code, I ran

  uv run ruff check && uv run ruff format && uv run mypy --install-types .

[github-actions]

1. High: behave tests will fail to import after this PR
   test/utils.py was moved to test/pytests/utils.py and the original module path was removed, but behave files still import test.utils:

   • environment.py
   • basic_commands.py
   • connection.py

   This introduces a regression for the BDD test suite (ModuleNotFoundError: test.utils).
   Action: either update those imports to test.pytests.utils or add a compatibility shim at test/utils.py that re-exports from test.pytests.utils.

2. Low: test coverage weakened for system command execution path
   In test_sqlexecute.py, test_system_command_output now uses system echo ... instead of reading a real file via cat.
   This no longer validates argument/path handling for external commands (including file path parsing).
   Action: keep the portable echo check if desired, but add a separate test using a temp file and an external command to preserve coverage of real command/path execution.

No new security issues identified in this diff.