Localise and decrease openfermion-dependency in qchem tests

[soranjh]

Context:
This PR localises the openfermion-dependent tests in one place and reduces openfermion dependency in qchem tests where possible.

Description of the Change:

• All openfermion-dependent tests are now located in of_tests.

• The new module test_structure is created to collect the tests of the qchem.structure module in one place. These tests were previously inside of_tests and had some openfermion dependency which is now removed.

• The rest of the qchem tests are brought inside tests.qchem.

Benefits:
Makes it easier to manage the external dependencies in the qchem tests.

Possible Drawbacks:

Related GitHub Issues:

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[codecov]

Codecov Report

Merging #2593 (70fbcaf) into master (5d5181f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2593   +/-   ##
=======================================
  Coverage   99.59%   99.59%           
=======================================
  Files         243      243           
  Lines       19630    19630           
=======================================
  Hits        19550    19550           
  Misses         80       80           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5d5181f...70fbcaf. Read the comment docs.

[obliviateandsurrender]

Looks good! Just left two unrelated comments. Also, any idea why doc build is failing? 🤔

Found out it is because of protobuf 4.21.0.

[soranjh]

Also, any idea why doc build is failing?

It is passing after a rerun.