Use UTF characters in the BMP in tests

ChromeDriver only supports characters in the BMP (Basic Multilingual
Plane). One test fails with ChromeDriver because it was entering emojis.

I'm using kanji characters instead, although I must admit I'm not sure
why such an unusual login was used in the first place.

spec/system/users_auth_spec.rb

@@ -46,11 +46,11 @@ describe "Users" do
       end
 
       scenario "Sign in with username" do
-        create(:user, username: "👻👽👾🤖", email: "ash@nostromo.dev", password: "xenomorph")
+        create(:user, username: "中村広", email: "ash@nostromo.dev", password: "xenomorph")
 
         visit "/"
         click_link "Sign in"
-        fill_in "user_login",    with: "👻👽👾🤖"
+        fill_in "user_login",    with: "中村広"
         fill_in "user_password", with: "xenomorph"
         click_button "Enter"