Optimize MinitestExplorationHelpers#test_cases helper

[moberegger]

A follow up to #363

Optimizes the MinitestExplorationHelpers#test_cases helper. The intent of this helper was to find test and it class definitions, but this would actually descend into method bodies, which incurs a rather substantial traversal cost. The optimization basically terminates the traversal earlier.

This does technically introduce a change in behavior. The old implementation could result in false positives/negatives where the proposed optimized version does not. For example, with something like

class FooTest < Minitest::Test
  def build_matrix(cases)
    cases.each do |name|
      it(name) { skip }
    end
  end
end

originally the cop would see it and think it's a valid test definition even though it is not. it is a class level DSL, and in this case an it instance method would be called. I am having trouble thinking of a good example here; I can't imagine this happening too often given that this is not valid syntax (at least as far as the minitest DSL is concerned), but I suppose there is a corner case here where the developer implemented their own it instance method helper or whatever that allowed this to run. I mention this mostly for completeness.

Benchmarks below. This was run against 5,000 cop executions per rule. Note that the Minitest/MultipleAssertions benchmark ran with the changes in #363 included.

Cop	Before	After	Speedup	Offenses
Minitest/MultipleAssertions	0.326s	0.256s	1.27x	3750
Minitest/NoAssertions	0.255s	0.190s	1.34x	250
Minitest/NoTestCases	0.159s	0.097s	1.64x	0
Minitest/DuplicateTestRun	0.253s	0.121s	2.08x	250
Minitest/NonPublicTestMethod	0.159s	0.096s	1.66x	250

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• 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).
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• 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.