Add neovim 0.6 to run-tests (#3998)

* Update test scripts
* Remove neovim 0.3 and 0.4
* Add neovim 0.6.1

Co-authored-by: Horacio Sanson <horacio@allm.inc>
Co-authored-by: w0rp <devw0rp@gmail.com>

test/linter/test_less_stylelint.vader

@@ -26,6 +26,7 @@ Execute(The global override should work):
 
 Execute(Extra options should be configurable):
   let b:ale_less_stylelint_options = '--whatever'
+  call ale#test#SetFilename('../test-files/dummy')
 
   AssertLinter 'stylelint',
   \ ale#Escape('stylelint') . ' --whatever --stdin-filename %s'