#2132 - lint and fix with ale#command#Run

A new function is added here which will later be modified for public use
in linter and fixer callbacks. All linting and fixing now goes through
this new function, to prove that it works in all cases.

test/test_command_chain.vader

@@ -1,6 +1,7 @@
 Before:
   Save &shell, g:ale_run_synchronously
   let g:ale_run_synchronously = 1
+  unlet! g:ale_run_synchronously_callbacks
 
   if !has('win32')
     set shell=/bin/sh
@@ -47,6 +48,7 @@ Before:
 
 After:
   Restore
+  unlet! g:ale_run_synchronously_callbacks
   unlet! g:first_echo_called
   unlet! g:second_echo_called
   unlet! g:final_callback_called
@@ -63,6 +65,7 @@ Given foobar (Some imaginary filetype):
 Execute(Check the results of running the chain):
   AssertEqual 'foobar', &filetype
   call ale#Queue(0)
+  call ale#test#FlushJobs()
 
   Assert g:first_echo_called, 'The first chain item was not called'
   Assert g:second_echo_called, 'The second chain item was not called'