Using Regexp.union to generate filter regexp

We can't just use filter = Regexp.union(method_names).inspect
because we need to anchor our regexp to avoid this kind of collisions:

  focus
  def test_method; end
  def test_method_with_smthg; end

Without anchors regexp (i.e. /TestClass\#test_method|TestClass\#test_method_with_smthg/)
will match both test methods.