Fix #2668 - Set g:ale_go_go111module

test/fixers/test_gomod_fixer_callback.vader

@@ -1,5 +1,6 @@
 Before:
   Save g:ale_go_go_executable
+  Save g:ale_go_go111module
 
   " Use an invalid global executable, so we don't match it.
   let g:ale_go_go_executable = 'xxxinvalid'
@@ -9,6 +10,8 @@ Before:
 After:
   Restore
 
+  unlet! b:ale_go_go111module
+
   call ale#test#RestoreDirectory()
 
 Execute(The gomod callback should return the correct default values):