Don't check if stderr empty in test_xdg_config_symlink (#12435)

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Fixes a problem introduced in
https://github.com/nushell/nushell/pull/12420 where one of the config
path tests (`test_xdg_config_symlink`) fails if, on MacOS, a developer
already has config files in the default location for that platform.

This happened because the test is making sure the
`xdg_config_home_invalid` error isn't reported, but to do that, it
asserts that stderr is empty, which it is not if in the case mentioned
above, because Nushell warns that the default location
(`~/.config`/`~/Library/Application Support`) is not empty but
`XDG_CONFIG_HOME` is empty.

If someone with a Mac could test this, that'd be great.

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src/tests/test_config_path.rs

@@ -278,6 +278,10 @@ fn test_xdg_config_symlink() {
         playground.symlink("real", config_link);
 
         let stderr = run_interactive_stderr(playground.cwd().join(config_link));
-        assert!(stderr.is_empty(), "stderr was {}", stderr);
+        assert!(
+            !stderr.contains("xdg_config_home_invalid"),
+            "stderr was {}",
+            stderr
+        );
     });
 }