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_sugarss_stylelint.vader

@@ -25,6 +25,8 @@ Execute(The global override should work):
   AssertLinter 'foobar', ale#Escape('foobar') . '  --syntax=sugarss --stdin-filename %s'
 
 Execute(Extra options should be configurable):
+  call ale#test#SetFilename('../test-files/dummy')
+
   let b:ale_sugarss_stylelint_options = '--configFile ''/absolute/path/to/file'''
 
   AssertLinter 'stylelint',