bazel: set size = small for all tests (backport #799)

[mergify]

🎉 New feature

Updates bazel metadata

Summary

The bazel size attribute specifies the resources required to run a test and its default timeout (see bazel test encyclopedia). All gz-math tests finished in less than 20 seconds in the most recent run that I observed, so I set all to have size = "small", which corresponds to a default timeout of 60 seconds.

Backport Policy

• This is safe to backport to the following versions:
  • Jetty
  • Ionic
  • Harmonic
  • Fortress
• This should not be backported
• I am not sure
• Other (fill in yourself)

Test it

Check that bazel CI passes

Checklist

• Signed all commits for DCO
• Added a screen capture or video to the PR description that demonstrates the feature
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• Updated Bazel files (if adding new files). Created an issue otherwise.
• While waiting for a review on your PR, please help review another open pull request to support the maintainers
• Was GenAI used to generate this PR? If so, make sure to add "Assisted-by" to your commits. (See this policy for more info.)

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by and Generated-by messages.

Backports: If this is a backport, please use Rebase and Merge instead.

---

This is an automatic backport of pull request #799 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of cdc10cb has failed:

On branch mergify/bp/gz-math8/pr-799
Your branch is up to date with 'origin/gz-math8'.

You are currently cherry-picking commit cdc10cb.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   eigen3/BUILD.bazel

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   BUILD.bazel

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[scpeters]

I manually resolved conflicts in 9de00c8, external review requested