[tests]: Make each unit test stand-alone

[t1m0thyj]

Is your feature or enhancement request related to a problem or limitation? Please describe

Currently, some of the unit tests in test_zowe_core.py that access the keyring cannot be run individually, because they rely on some setup to be done by an earlier test.

Describe your enhancement idea

Each test should perform all of the necessary setup.

Describe alternatives you've considered

Running all the tests in the file works, but may be less convenient.

Provide any additional context

An example of a test that currently fails when run standalone is test_profile_loading_exception - thanks @pem70

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