add --download-only option to openfe test

[atravitz]

this will allow us to:

• pull all the test data up-front, so that we fail fast if there's a zenodo-fetching issue
• enable users to pull data on a instance with internet access, then run tests on air-gapped compute
• both cli and openfe zenodo info is stored in a registry - this should make it easier to migrate to cloudflare in the future, plus we can maybe diff that file to trigger a cache update for CI

Checklist

• All new code is appropriately documented (user-facing code must have complete docstrings).
• Added a news entry, or the changes are not user-facing.
• Ran pre-commit: you can run pre-commit locally or comment on this PR with pre-commit.ci autofix.

Manual Tests: these are slow so don't need to be run every commit, only before merging and when relevant changes are made (generally at reviewer-discretion).

• GPU integration tests
• example notebook testing
• packaging tests: run this for any large feature PRs or PRs that add test data.

Developers certificate of origin

• I certify that this contribution is covered by the MIT License here and the Developer Certificate of Origin at https://developercertificate.org/.

[codecov]

Codecov Report

❌ Patch coverage is 82.19178% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.98%. Comparing base (b81c963) to head (e00863a).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/openfe/data/_downloader.py	25.00%	9 Missing ⚠️
src/openfecli/commands/test.py	70.00%	3 Missing ⚠️
src/openfe/tests/protocols/conftest.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1814      +/-   ##
==========================================
- Coverage   95.27%   92.98%   -2.30%     
==========================================
  Files         201      204       +3     
  Lines       17580    17623      +43     
==========================================
- Hits        16750    16386     -364     
- Misses        830     1237     +407     

Flag	Coverage Δ
fast-tests	92.98% <82.19%> (?)
slow-tests	?

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[atravitz]

note: we moved to from figshare to zenodo here:

openfe/src/openfe/tests/protocols/test_openmmutils.py

Lines 1037 to 1038 in f7561cd

	RFE_OUTPUT = pooch.create(
	path=POOCH_CACHE,

but this was left as a stragger no-op.

[mikemhenry]

LGTM, a hash of the _registry.py files can now be used as the cache key, this will also make it easy to migrate things somewhere else since we only have one file to track.

[github-actions]

🚨 API breaking changes detected! 🚨
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