Package plugin test suites + a [tests] extra so downstream builders can run them (lehrer plugin-regression)

Summary

Ship each maintained ol-openedx-* plugin's test suite inside its built
distribution and declare a tests extra bundling its test-only dependencies,
so the tests can be installed and run against a real, already-built Open edX
platform via pip install "<dist>[tests]==<version>".

Why

lehrer added a plugin-regression runner (dagger call platform plugin-regression, lehrer#111)
that runs each installed plugin's own pytest suite inside a built cell image
(the real edx-platform version + the deployment's pinned plugin set + aqueduct
settings). It uses pytest discovery: it runs whatever tests are installed in
the image, resolving packages at runtime.

Today it discovers nothing, because:

• published plugin wheels/sdists ship no test files (verified: the
  ol-openedx-logging==0.3.5 sdist contains only settings/test.py, no
  tests/), and
• this monorepo has no per-package tags, so there's no way for a downstream
  builder to fetch "the tests at the exact installed version."

lehrer already installs "<dist>[tests]==<pinned-version>" opportunistically —
which is a safe no-op until the extra exists — so the moment a plugin ships its
tests + a tests extra, its suite starts running in that pipeline with no
lehrer change required.

What to do (per maintained plugin)

• Include the plugin's tests/ in the built distribution (package data /
  don't exclude tests), so pip install <dist> lands the tests in
  site-packages where pytest --pyargs <module> can discover them.
• Declare a tests optional-dependency extra with the test-only deps the
  suite needs (pytest-mock, responses, factory_boy, etc.), matching what
  run_edx_integration_tests.sh installs today.
• Confirm the suite runs under --ds=lms.envs.test (or the plugin's own
  standalone settings) against an installed edx-platform.

Roll out incrementally — one plugin unblocks real signal for that plugin.

Verify

Once at least one plugin ships the extra, from a lehrer checkout:

lehrer build plugin-regression --cell mit-ol/master/mitxonline \
  --custom-settings ./deployments/mit-ol/settings

should collect and run that plugin's suite (instead of "no tests discovered").

Tracked in the lehrer CI-verification project; companion CI-wiring task is
blocked on this.

Start by selecting one maintained ol-openedx-* plugin and inspect its packaging metadata, tests/ directory, and run_edx_integration_tests.sh to compare current test dependencies. Build and install that plugin with its tests extra, then run the documented lehrer build plugin-regression command and confirm pytest collects and runs the installed suite against the platform.