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

test/fixers/test_yapf_fixer_callback.vader

@@ -4,7 +4,7 @@ Before:
   " Use an invalid global executable, so we don't match it.
   let g:ale_python_yapf_executable = 'xxxinvalid'
 
-  silent! execute 'cd /testplugin/test/command_callback'
+  call ale#test#SetDirectory('/testplugin/test/fixers')
   silent cd ..
   silent cd command_callback
   let g:dir = getcwd()
@@ -12,11 +12,7 @@ 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 yapf callback should return the correct default values):
   AssertEqual