Allow args in SpecRunner task

tasks/spec_runner.rake

@@ -11,7 +11,10 @@ module RuboCop
   # The specs will be run in parallel if the system implements `fork`.
   # If ENV['COVERAGE'] is truthy, code coverage will be measured.
   class SpecRunner
-    def initialize(external_encoding: 'UTF-8', internal_encoding: nil)
+    attr_reader :rspec_args
+
+    def initialize(rspec_args = %w[spec], external_encoding: 'UTF-8', internal_encoding: nil)
+      @rspec_args = rspec_args
       @previous_external_encoding = Encoding.default_external
       @previous_internal_encoding = Encoding.default_internal
 
@@ -20,8 +23,6 @@ module RuboCop
     end
 
     def run_specs
-      rspec_args = %w[spec]
-
       n_failures = with_encoding do
         if Process.respond_to?(:fork)
           parallel_runner_klass.new(rspec_args).execute