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_vim_vimls.vader

@@ -70,7 +70,8 @@ Execute(should let the global executable to be used):
   AssertLinter 'vim-language-server',
   \ ale#Escape('vim-language-server') . ' --stdio'
 
-Execute(should let the executable to be configured):
+Execute(should allow the executable to be configured):
   let g:ale_vim_vimls_executable = 'foobar'
+  call ale#test#SetFilename('../test-files/dummy')
 
   AssertLinter 'foobar', ale#Escape('foobar') . ' --stdio'