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.

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`.