New cop to disallow before and a single test case

[fatkodima]

It makes little sense to write the test code like

context 'when something' do
  before do
    # some setup
  end

  it 'tests something' do # <--- only one test case
    # some testing
  end
end

The test is now unnecessary segregated. When more before blocks precedes it outside of the context it is even harder to combine all these pieces.

Better is to write it like

context 'when something' do
  it 'tests something' do
    # some setup
    # some testing
  end
end