Consider the multibyte value in the method name of system test

`String#[]` returns a value that considers multibyte value. But some
file systems use byte for maximum filename length. So if applications
use that file system and multibyte value to a method name, currently
check doesn't work expected.

This PR fixes to use `String#byteslice` instead of `String#[]`. Also,
added `String#scrub` to avoid generating an invalid byte sequence.

lib/rspec/rails/example/system_example_group.rb

@@ -38,10 +38,18 @@ def passed?
 
       # @private
       def method_name
-        @method_name ||= [
-          self.class.name.underscore,
-          RSpec.current_example.description.underscore
-        ].join("_").tr(CHARS_TO_TRANSLATE.join, "_")[0...200] + "_#{rand(1000)}"
+        @method_name ||= begin
+          m = [
+            self.class.name.underscore,
+            RSpec.current_example.description.underscore
+          ].join("_").tr(CHARS_TO_TRANSLATE.join, "_")
+
+          if RbConfig::CONFIG["host_os"] =~ /linux/
+            m.byteslice(0...200).scrub("") + "_#{rand(1000)}"
+          else
+            m[0...200] + "_#{rand(1000)}"
+          end
+        end
       end
 
       # Delegates to `Rails.application`.

spec/rspec/rails/example/system_example_group_spec.rb

@@ -16,6 +16,17 @@ module RSpec::Rails
             expect(example.send(:method_name)).to start_with('method_name')
           end
         end
+
+        it 'slices long method name' do
+          group = RSpec::Core::ExampleGroup.describe ActionPack do
+            include SystemExampleGroup
+          end
+
+          example = group.new
+          example_class_mock = double('name' => "#{'あ'*100}")
+          allow(example).to receive(:class).and_return(example_class_mock)
+          expect(example.send(:method_name).bytesize).to be <= 210
+        end
       end
 
       describe '#driver' do