Split test_problems.py into test_*.py for each class

[Newtech66]

Description

• Split test_problems.py into several test_*.py files which each contain the tests for a particular problem class.
• Resolves Improve test_problems.py #226

Checklist

• I have performed a self-review of my code.
• I have commented my code and used numpy-style docstrings
• I have made corresponding updates to the documentation.
• My changes generate no new warnings
• I have added/updated tests to make sure bugfix/feature works.
• New and existing unit tests pass locally with my changes.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

PortfolioOptimization has been added to test_QUBO.py, but it doesn't pass on my system because the change to helper_functions.py isn't reflecting on the install for some reason.

[vishal-ph]

@Newton66, can you please update your branch by pulling in the latest changes from dev. We realized there was an issue with the test_ext_dev action script that prevented the tests inside src/*/tests/ folders to run. We have fixed this in the latest commit on dev. Once you have pulled these changes, we can run the tests again

[Newtech66]

@vishal-ph I did it

[vishal-ph]

@Newtech66, I cannot see the new changes on your branch yet, could you please merge the branch once more? Thanks

[vishal-ph]

The only test that failed in this PR was due to IBM account credentials being inaccessible by forked PRs. The PR is good to merge