Add cop to avoid focus

.rubocop.yml

@@ -1,5 +1,6 @@
 require:
   - ./cop/none_without_block_cop
+  - ./cop/no_focus_cop
 
 AllCops:
   DisabledByDefault: true

cop/no_focus_cop.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+require 'rubocop'
+
+module Cop
+  # Make sure no tests are focused, from https://github.com/rubocop-hq/rubocop/issues/3773#issuecomment-420662102
+  class NoFocusCop < RuboCop::Cop::Cop
+    MSG = 'Remove `focus` from tests.'
+
+    def_node_matcher :focused?, <<-MATCHER
+      (send nil? :focus)
+    MATCHER
+
+    def on_send(node)
+      return unless focused?(node)
+
+      add_offense node
+    end
+  end
+end