[vWIP] expand Network fixtures for consistent test setup

[SatvikSaluja]

This PR expands the Network fixtures in conftest.py to support multiple network configurations (default, no drives, small mesh, and 3x3 mesh).

It also introduces a shared default_params fixture to avoid duplicated parameter-loading logic.

Part of #1118.

[asoplata]

Please see my comment #1247 (comment)

[asoplata]

Hello Satvik,

Since we did not receive a GSOC contributor for our test-refactoring project, I would like to "reactivate" the three PRs that you made that began some of the work, including #1247, #1251, and #1257. You would not be obligated to continue contributing to them since you'll be working on the Synapse project, but this way you still get credit for the work you've already done on them.

Would you please recreate the branch used for this PR (test-expand-network-fixtures) and push to your origin, so that I can reopen this PR? If you deleted your local copy of the branch, that's fine, you can checkout a version I saved here on my own fork https://github.com/asoplata/hnn-core/tree/test-expand-network-fixtures . I recommend that you add my fork as a remote in your local git install, fetch my branches, checkout that specific branch, make a new branch of the same name, then push that new branch to your origin. I believe that will enable us to reopen this PR.