New cop: Minitest/NoTestCases

[fatkodima]

Warns when there are not test cases in the test class. Similar to the existing NoAssertions cop.

# bad
class SomethingTest < Minitest::Test
  def setup; end
end

# good
class SomethingTest < Minitest::Test
  def setup; end

  def test_something
    assert something
  end
end