[wic][PATCH v2 1/9] tests: add the standalone test-suite skeleton

wic currently has no test mechanism of its own; it relies on the
oe-selftest from oe-core for all its testing, which means a full
bitbake build is needed to exercise even pure-Python logic. This
commit lays the groundwork for a small standalone suite that runs
from a plain checkout with nothing but pytest, so that logic can be
pinned down and kept stable as the code evolves.

This commit adds the skeleton only; it does not add any tests.

What this adds:

  - pyproject.toml: a "tests" optional-dependency group (pytest only)
    so that "pip install -e .[tests]" pulls in what the suite needs,
    plus a [tool.pytest.ini_options] section. The pytest options keep
    a test's scratch directory only when that test fails, so repeated
    runs do not accumulate leftover files.

  - tests/unit/ and tests/docs/: the directories that hold the
    in-process unit suites and the suite documentation. Git cannot
    track an empty directory, so a placeholder .gitkeep is committed
    in each to create it. The .gitkeep files have no content and are
    removed once real files land in those directories.

  - .gitignore: ignore the .pytest_cache/ directory that pytest
    writes at the top of the checkout.

  - README.md: a Testing section pointing at the suite and at
    tests/docs/, and a tests/* entry in the project layout list.

AI-Generated: codex/claude-opus 4.7 (xhigh)
Signed-off-by: Trevor Woerner <[email protected]>
---
changes in v2:
- v1 submitted the entire test suite as a single commit; v2 breaks
  the work into a reviewable series, and this patch is one step of it.
---
 .gitignore          |  3 +++
 README.md           | 14 ++++++++++++++
 pyproject.toml      | 12 ++++++++++++
 tests/docs/.gitkeep |  0
 tests/unit/.gitkeep |  0
 5 files changed, 29 insertions(+)
 create mode 100644 tests/docs/.gitkeep
 create mode 100644 tests/unit/.gitkeep

diff --git a/.gitignore b/.gitignore
index eeb8a6ec4087..07992096c0fb 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1 +1,4 @@
 **/__pycache__
+
+# pytest cache
+/.pytest_cache/
diff --git a/README.md b/README.md
index 75229421763c..497ebb1de0f0 100644
--- a/README.md
+++ b/README.md
@@ -46,6 +46,20 @@ environment file or folder (generated via `bitbake -c rootfs_wicenv
 - `src/wic/*`: core engine, plugins, and helpers.
 - `src/bb/*`: various bitbake helpers that were brought along and used in other parts of wic.
 - `src/oe/*`: various oe-core helpers that were brought along and used in other parts of wic.
+- `tests/*`: the standalone test suite and its documentation (see Testing below).
+
+## Testing
+
+wic ships a standalone test suite under `tests/` that runs from a plain
+checkout, with no bitbake and no OpenEmbedded build required. The test
+extras pull in everything the suite needs:
+
+```
+pip install -e ".[tests]"
+```
+
+See [tests/docs/](tests/docs/) for how to run the suite and the
+conventions it follows.
 
 ## Contributing
 
diff --git a/pyproject.toml b/pyproject.toml
index fdc1ce0f5ece..d660e39007c4 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -21,6 +21,11 @@ classifiers = [
 Homepage = "https://git.yoctoproject.org/wic"
 Repository = "https://git.yoctoproject.org/wic"
 
+[project.optional-dependencies]
+tests = [
+    "pytest >= 7.0",
+]
+
 [project.scripts]
 wic = "wic.cli:main"
 
@@ -36,3 +41,10 @@ build-backend = "hatchling.build"
 
 [tool.hatch.version]
 path = "src/wic/cli.py"
+
+[tool.pytest.ini_options]
+# Keep a test's scratch directory only when it fails; a passing test's
+# tmp_path is removed automatically so repeated runs do not accumulate
+# leftover files under the pytest base temp directory.
+tmp_path_retention_policy = "failed"
+tmp_path_retention_count  = 1
diff --git a/tests/docs/.gitkeep b/tests/docs/.gitkeep
new file mode 100644
index 000000000000..e69de29bb2d1
diff --git a/tests/unit/.gitkeep b/tests/unit/.gitkeep
new file mode 100644
index 000000000000..e69de29bb2d1
-- 
2.50.0.173.g8b6f19ccfc3a