Consider the multibyte value in the method name of system test

`String#[]` returns a value that considers multibyte value. But maximum
filename length use bytes. So if applications use 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.

features/system_specs/system_specs.feature

@@ -107,3 +107,29 @@ Feature: System spec
         Then the output should contain "1 example, 0 failures"
         And the output should not contain "starting Puma"
         And the exit status should be 0
+
+    @system_test
+    Scenario: System specs can save screenshots if the method name is too long
+        Given a file named "spec/system/widget_system_spec.rb" with:
+          """ruby
+          require "rails_helper"
+
+          RSpec.describe "Widget management", :type => :system do
+            before do
+              driven_by(:selenium_chrome_headless)
+            end
+
+            it "long test name #{'あ'*100}" do
+              visit "/widgets/new"
+
+              fill_in "Name", :with => "My Widget"
+              click_button "Create Widget"
+
+              expect(page).to have_text("Widget was created.")
+            end
+          end
+          """
+        When I run `rspec spec/system/widget_system_spec.rb`
+        Then the output should contain "1 example, 1 failure"
+        And the output should not contain "Errno::ENAMETOOLONG"
+        And the output should contain "[Screenshot]"

lib/rspec/rails/example/system_example_group.rb

@@ -41,7 +41,7 @@ def method_name
         @method_name ||= [
           self.class.name.underscore,
           RSpec.current_example.description.underscore
-        ].join("_").tr(CHARS_TO_TRANSLATE.join, "_")[0...200] + "_#{rand(1000)}"
+        ].join("_").tr(CHARS_TO_TRANSLATE.join, "_").byteslice(0...200).scrub("") + "_#{rand(1000)}"
       end
 
       # Delegates to `Rails.application`.