Add return type for tests

[dstrain115]

• Adds return type for tests as None.
• Looking at Pass --disallow-untyped-defs flag to mypy. #4393, it occured we could automate a bunch of these errors with a simple script by adding "-> None" to tests.
• At first, I thought this was ridiculous, but actually, this enables mypy to check the tests for typing errors, of which there are many.
• These are the files that can be automatically changed that do not having typing errors.
• The only changes are adding "-> None" to the test functions.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.67%. Comparing base (b136ae8) to head (f9f37c5).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7327      +/-   ##
==========================================
- Coverage   98.67%   98.67%   -0.01%     
==========================================
  Files        1111     1111              
  Lines       96811    96811              
==========================================
- Hits        95528    95524       -4     
- Misses       1283     1287       +4     

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[vtomole]

I didn't look through every file: It is a lot of tests 😅, but if a script did this then I believe it's fine.

[vtomole]

Do we want to add this commit to https://github.com/quantumlib/Cirq/blob/main/.git-blame-ignore-revs?