Clean up tests to stop people copy and pasting the wrong examples

test/handler/test_nix_handler.vader

@@ -1,6 +1,10 @@
-Execute(The nix handler should parse nix-instantiate error messages correctly):
+Before:
   runtime ale_linters/nix/nix.vim
 
+After:
+  call ale#linter#Reset()
+
+Execute(The nix handler should parse nix-instantiate error messages correctly):
   AssertEqual
   \ [
   \   {
@@ -22,6 +26,3 @@ Execute(The nix handler should parse nix-instantiate error messages correctly):
   \   'error: syntax error, unexpected IN, at /path/to/filename.nix:23:14',
   \   'error: syntax error, unexpected ''='', expecting '';'', at /path/to/filename.nix:3:12',
   \ ])
-
-After:
-  call ale#linter#Reset()