Fix some escaping and make some tests set filenames consistently

autoload/ale/test.vim

@@ -0,0 +1,19 @@
+" Author: w0rp <devw0rp@gmail.com>
+" Description: Functions for making testing ALE easier.
+"
+" This file should not typically be loaded during the normal execution of ALE.
+
+" Change the filename for the current buffer using a relative path to
+" the script without running autocmd commands.
+"
+" If a g:dir variable is set, it will be used as the path to the directory
+" containing the test file.
+function! ale#test#SetFilename(path) abort
+    let l:dir = get(g:, 'dir', '')
+
+    if empty(l:dir)
+        let l:dir = getcwd()
+    endif
+
+    silent noautocmd execute 'file ' . fnameescape(simplify(l:dir . '/' . a:path))
+endfunction