[ENH] Implemented MockCluster in testing module

[notaryanramani]

Reference Issues/PRs

Fixes #1529

What does this implement/fix? Explain your changes.

Implements MockCluster class inline with classification and regression, along with test cases.

Does your contribution introduce a new dependency? If yes, which one?

No

Any other comments?

No

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[chrisholder]

LGTM! Thanks for the addition!