Fix undefined method error after previous change

After the previous change, one would get the error `undefined method
`mode' for Byebug:Module` when running a file with `byebug` calls. A
`require` call needed to be moved up.

lib/byebug/attacher.rb

@@ -8,8 +8,6 @@ module Byebug
   # Starts byebug, and stops at the first line of user's code.
   #
   def self.attach
-    require "byebug/core"
-
     unless started?
       self.mode = :attached
 
@@ -35,6 +33,8 @@ def self.spawn(host = "localhost", port = nil)
 #
 module Kernel
   def byebug
+    require "byebug/core"
+
     Byebug.attach unless Byebug.mode == :off
   end
 

test/runner_against_program_with_byebug_call_test.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+module Byebug
+  #
+  # Tests standalone byebug when debugging a target program
+  #
+  class RunnerAgainstProgramWithByebugCallTest < TestCase
+    def setup
+      super
+
+      example_file.write("require 'byebug'\nbyebug\nsleep 0")
+      example_file.close
+    end
+
+    def test_run_with_a_script_to_debug
+      stdout = run_program(
+        ["ruby", example_path],
+        'puts "Program: #{$PROGRAM_NAME}"'
+      )
+
+      assert_match(/Program: #{example_path}/, stdout)
+    end
+  end
+end