Fix test_history_path_detection panic: call test_init()

test_history_path_detection calls add_pending_with_file_detection(),
which spawns a thread pool task via ThreadPool::perform(). This
requires threads::init() to have been called, otherwise
assert_is_background_thread() panics.

Add the missing test_init() call, matching other tests that use
subsystems requiring initialization.

Closes #12604

src/history/history.rs

@@ -1810,6 +1810,7 @@ mod tests {
     use crate::env::{EnvMode, EnvSetMode, EnvStack};
     use crate::fs::{LockedFile, WriteMethod};
     use crate::prelude::*;
+    use crate::tests::prelude::test_init;
     use fish_build_helper::workspace_root;
     use fish_wcstringutil::{
         string_prefixes_string, string_prefixes_string_case_insensitive, wcs2bytes,
@@ -2292,6 +2293,7 @@ fn test_history_merge() {
 
     #[test]
     fn test_history_path_detection() {
+        let _cleanup = test_init();
         // Regression test for #7582.
         // Temporary directory for the history files.
         let hist_tmpdir = fish_tempfile::new_dir().unwrap();