Add new Minitest/AssertPredicate and Minitest/RefutePredicate cops

[koic]

This PR adds new Minitest/AssertPredicate and Minitest/RefutePredicate cops.

Minitest/AssertPredicate cop

This cop enforces the test to use assert_predicate
instead of using assert(obj.a_predicate_method?).

# bad
assert(obj.one?)
assert(obj.one?, 'message')

# good
assert_predicate(obj, :one?)
assert_predicate(obj, :one?, 'message')

Minitest/RefutePredicate cop

This cop enforces the test to use refute_predicate
instead of using refute(obj.a_predicate_method?).

# bad
refute(obj.one?)
refute(obj.one?, 'message')

# good
refute_predicate(obj, :one?)
refute_predicate(obj, :one?, 'message')

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.