Disable UnusedPrivateMethod detector by default.

[troessner]

Fixes #844

Regarding the "re-enable a by default deactivated smell detector for tests" I took some inspiration from the corresponding Attribute smell detector PR here

[troessner]

Due to the way we load SmellDetectors and Examiners something like:

  before do
    allow(described_class).
      to receive(:default_config).
      and_return(described_class.superclass.default_config)
  end

or

  before do
    described_class.instance_eval { def default_config; super; end }
  end

doesn't work. But given that we don't do this every other week I'd consider this good enough given the task at hand.