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_cpp_cppcheck.vader

@@ -5,7 +5,7 @@ Before:
 After:
   " Remove a test file we might open for some tests.
   if &buftype != 'nofile'
-    :q!
+    set nomodified
     set buftype=nofile
   endif