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_rust_rls.vader

@@ -17,13 +17,16 @@ Execute(The toolchain should be ommitted if not given):
 
   AssertLinter 'rls', ale#Escape('rls')
 
-Execute(The project root should be detected correctly):
-  AssertLSPProject ''
-
+Execute(The project root should be detected correctly for cargo projects):
   call ale#test#SetFilename('../test-files/rust/cargo/testfile.rs')
 
   AssertLSPProject ale#path#Simplify(g:dir . '/../test-files/rust/cargo')
 
+Execute(The project root should be empty when no project files can be detected):
+  call ale#test#SetFilename('../test-files/dummy')
+
+  AssertLSPProject ''
+
 Execute(Should accept configuration settings):
   AssertLSPConfig {}
   let b:ale_rust_rls_config = {'rust': {'clippy_preference': 'on'}}