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_pyflakes.vader

@@ -34,6 +34,7 @@ Execute(You should be able to override the pyflakes virtualenv lookup):
 
 Execute(Setting executable to 'pipenv' appends 'run pyflakes'):
   let g:ale_python_pyflakes_executable = 'path/to/pipenv'
+  call ale#test#SetFilename('../test-files/dummy')
 
   AssertLinter 'path/to/pipenv',
   \ ale#Escape('path/to/pipenv') . ' run pyflakes %t',