Add unit tests for backtrace_filter
user
```ruby
require 'cucumber/formatter/backtrace_filter'

module Cucumber
  module Formatter
    describe BacktraceFilter do
      context '#exception' do
        before do
          trace = %w(a b
                     _anything__/vendor/rails__anything_
                     _anything__lib/cucumber__anything_
                     _anything__bin/cucumber:__anything_
                     _anything__lib/rspec__anything_
                     _anything__gems/__anything_
                     _anything__minitest__anything_
                     _anything__test/unit__anything_
                     _anything__.gem/ruby__anything_
                     _anything__lib/ruby/__anything_
                     _anything__bin/bundle__anything_)
          @exception = Exception.new
          @exception.set_backtrace(trace)
        end

        it 'filters unnecessary traces' do
          BacktraceFilter.new(@exception).exception
          expect(@exception.backtrace).to eql %w(a b)
        end
      end
    end
  end
end


```
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