functests: Refactor 009_bufleave_autocommand_spec

It was not testing anything actually: the `e yy` command simply failed because 
of unsaved changes, BufLeave never run thus.

test/functional/legacy/009_bufleave_autocommand_spec.lua

@@ -1,19 +1,27 @@
 -- Test for Bufleave autocommand that deletes the buffer we are about to edit.
 
 local helpers = require('test.functional.helpers')(after_each)
-local clear, insert = helpers.clear, helpers.insert
+
-local execute, expect = helpers.execute, helpers.expect
+local eq = helpers.eq
+local clear = helpers.clear
+local meths = helpers.meths
+local expect = helpers.expect
+local command = helpers.command
+local exc_exec = helpers.exc_exec
+local curbufmeths = helpers.curbufmeths
 
 describe('BufLeave autocommand', function()
   setup(clear)
 
   it('is working', function()
-    insert([[
+    meths.set_option('hidden', true)
-      start of test file xx
+    curbufmeths.set_lines(0, 1, false, {
-      end of test file xx]])
+      'start of test file xx',
+      'end of test file xx'})
 
-    execute('au BufLeave * bwipe yy')
+    command('autocmd BufLeave * bwipeout yy')
-    execute('e yy')
+    eq('Vim(edit):E143: Autocommands unexpectedly deleted new buffer yy',
+       exc_exec('edit yy'))
 
     expect([[
       start of test file xx