ZNE options validation fix

[gadial]

Summary

This PR fixes some error in the way ZNE options are validated and tested.

Details and comments

Currently, ZNE validation and testing have the following problems:

1. The _validate_zne_noise_factors method passes in the case the noise factors are the empty list [] although this is an invalid input, since the initial check if self.extrapolator and self.noise_factors considers self.noise_factors to be False in case it is an empty list.
2. ZNE tests pass for wrong reason. e.g. for {"resilience": {"zne": {"noise_factors": [1]}}} the test_bad_inputs test succeeds because ValidationError is thrown even though the intended error (not enough noise factors) is not tested, since this error is masked by the error of not having zne_mitigation: True in the resilience dictionary.

These problems are handled in the following manner:

1. _validate_zne_noise_factors now never skips. If noise factors or extrapolators are missing, it uses the default values described in the class's file.
2. test_bad_inputs now uses assertRaisesRegex with relevant error message for each test case, to avoid passing when irrelevant exceptions are thrown.
3. Another tests were added to test_bad_inputs for the case of empty noise factors list and for the case of missing zne_mitigation: True.

[coveralls]

Pull Request Test Coverage Report for Build 8603463800

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Details

• 15 of 15 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 83.488%

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Totals
Change from base Build 8578891638:	0.04%
Covered Lines:	6224
Relevant Lines:	7455

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💛 - Coveralls