Add initial pytest unit-test suite (+ CI)

[potato-20]

The repository currently ships without any automated tests. This PR introduces a tests/ package and a CI workflow to run it, without changing any production code.

What this adds

34 unit tests covering the host-side, dependency-free Python helpers:

Module	Covered
reversec.common.list	chunk, flatten (incl. strings-are-not-flattened)
reversec.common.text	indent, wrap (newline preservation)
reversec.common.fs	read / write / touch round-trips, missing-file handling
drozer.util	parse_server parsing, StoreZeroOrTwo 0/2-arg validation
drozer.android.Intent	validity rules, default attributes, flag combination

Tooling

• pyproject.toml: a [tool.pytest.ini_options] section with pythonpath = ["src"] so the suite runs with a bare pytest (no editable install needed), plus a test optional-dependency.
• .github/workflows/tests.yml: runs pytest across Python 3.9–3.13 on every push and pull request.

Notes

• The tests deliberately exercise only modules that import cleanly without the Twisted/protobuf runtime, so CI stays fast and does not depend on a full package build.
• No production code is modified — this is purely additive groundwork that future bug-fix PRs can extend with regression tests.

$ pytest -q
..................................                                       [100%]
34 passed in 0.05s