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_rustc.vader

@@ -5,9 +5,13 @@ After:
   call ale#assert#TearDownLinterTest()
 
 Execute(The default command should be correct):
+  call ale#test#SetFilename('../test-files/dummy')
+
   AssertLinter 'rustc', 'rustc --error-format=json --emit=mir -o /dev/null -'
 
 Execute(The options should be configurable):
+  call ale#test#SetFilename('../test-files/dummy')
+
   let b:ale_rust_rustc_options = '--foo'
 
   AssertLinter 'rustc', 'rustc --error-format=json --foo -'