Ensure no unhandled warnings are emitted during pytest run

[lbianchi-lbl]

Resolves: #1508

Changes proposed in this PR:

• Address warnings emitted by pytest for improper use
• Add handling for known warnings emitted by 3rd-party code (e.g. parameter_sweep) when running tests
• Add -Werror to default pytest flags used in CI to cause failure if new unhandled warnings appear
• Misc fixes to pytest.ini and test files

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