Fix concurrent setlocale() in string escape tests

In our C++ implementation, these tests were run serially.  As pointed out in
https://github.com/fish-shell/fish-shell/issues/11254#issuecomment-2735623229
we run them in parallel now, which means that one test could be changing
the global locale used by another.

In theory this could be fine because all tests are setting setting the
global locale to the same thing but the existence of a lock suggests that
setlocale() is not guaranteed to be atomic, so it's possible that another
thread uses a temporarily-invalid locale.

Fixes #11254

src/tests/string_escape.rs

@@ -1,3 +1,5 @@
+use std::sync::MutexGuard;
+
 use crate::common::{
     escape_string, str2wcstring, unescape_string, wcs2string, EscapeFlags, EscapeStringStyle,
     UnescapeStringStyle, ENCODE_DIRECT_BASE, ENCODE_DIRECT_END,
@@ -10,21 +12,21 @@
 
 /// wcs2string is locale-dependent, so ensure we have a multibyte locale
 /// before using it in a test.
-fn setlocale() {
-    let _guard = LOCALE_LOCK.lock().unwrap();
+fn setlocale() -> MutexGuard<'static, ()> {
+    let guard = LOCALE_LOCK.lock().unwrap();
 
     #[rustfmt::skip]
     const UTF8_LOCALES: &[&str] = &[
         "C.UTF-8", "en_US.UTF-8", "en_GB.UTF-8", "de_DE.UTF-8", "C.utf8", "UTF-8",
     ];
     if crate::libc::MB_CUR_MAX() > 1 {
-        return;
+        return guard;
     }
     for locale in UTF8_LOCALES {
         let locale = std::ffi::CString::new(locale.to_owned()).unwrap();
         unsafe { libc::setlocale(libc::LC_CTYPE, locale.as_ptr()) };
         if crate::libc::MB_CUR_MAX() > 1 {
-            return;
+            return guard;
         }
     }
     panic!("No UTF-8 locale found");
@@ -100,7 +102,7 @@ fn test_escape_var() {
 }
 
 fn escape_test(escape_style: EscapeStringStyle, unescape_style: UnescapeStringStyle) {
-    setlocale();
+    let _locale_guard = setlocale();
     let seed: u128 = 92348567983274852905629743984572;
     let mut rng = get_seeded_rng(seed);
 
@@ -174,7 +176,7 @@ fn str2hex(input: &[u8]) -> String {
 /// string comes back through double conversion.
 #[test]
 fn test_convert() {
-    setlocale();
+    let _locale_guard = setlocale();
     let seed = get_rng_seed();
     let mut rng = get_seeded_rng(seed);
     let mut origin = Vec::new();