Reduce array alloc on mount

[ericproulx]

#2529 change the @setup variable to an [] but the code is using += when adding a new step. This creates a new [] everytime and its not the best in terms on memory footprint. This PR replaced the += by << to not create new arrays.

I had to remove a test since having duplicates is expected. It would have failed in #2529 if the test had been like this since @setup is not the same object.

it 'does not unnecessarily retain duplicate setup blocks' do
  subject.logger
  first_setup = subject.instance_variable_get(:@setup)
  subject.logger
  last_setup = subject.instance_variable_get(:@setup)
  expect(first_setup.size).to eq(last_setup.size)
end

Running benchmark/remounting.rb shows the efficiency. It shows the difference between a Set and an Array but we still see a difference between << and +=
This is currently

Calculating -------------------------------------
 setup size: 1000
         using Array    19.623M memsize (   226.006k retained)
                       116.426k objects (     2.123k retained)
                        50.000  strings (     7.000  retained)
 setup size: 1
           using Set    11.680M memsize (     4.024k retained)
                       117.004k objects (    16.000  retained)
                         5.000  strings (     1.000  retained)

Comparison:
           using Set:   11680280 allocated
         using Array:   19623445 allocated - 1.68x more

This is with the PR

Calculating -------------------------------------
 setup size: 1000
         using Array    11.815M memsize (   229.806k retained)
                       112.426k objects (     2.123k retained)
                        50.000  strings (     7.000  retained)
 setup size: 1
           using Set    11.680M memsize (     4.024k retained)
                       111.004k objects (    16.000  retained)
                         5.000  strings (     1.000  retained)

Comparison:
           using Set:   11680280 allocated
         using Array:   11814541 allocated - 1.01x more