Make every test set filenames and switch directories in the same way, and fix some missing escaping for the rubocop linter

test/fixers/test_autopep8_fixer_callback.vader

@@ -6,7 +6,7 @@ Before:
   let g:ale_python_autopep8_executable = 'xxxinvalid'
   let g:ale_python_autopep8_options = ''
 
-  silent! execute 'cd /testplugin/test/command_callback'
+  call ale#test#SetDirectory('/testplugin/test/fixers')
   silent cd ..
   silent cd command_callback
   let g:dir = getcwd()
@@ -14,11 +14,7 @@ Before:
 After:
   Restore
 
-  silent execute 'cd ' . fnameescape(g:dir)
-  " Set the file to something else,
-  " or we'll cause issues when running other tests
-  silent file 'dummy.py'
-  unlet! g:dir
+  call ale#test#RestoreDirectory()
 
 Execute(The autopep8 callback should return the correct default values):
   AssertEqual