[Fix #1155] Add new Rails/AssertNotPredicate

[Earlopain]

Fixes #1155

This converts assert_not(obj.one?) into assert_not_predicate(obj, :one?) These methods are rails-specific and not present by default in minitest.

For users that prefer the refute method calls, Rails/RefuteMethods will correct this if configured that way.

Both the code and tests are pretty much a one-to-one copy from rubocop-minitest RefutePredicate

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• Added tests.
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