test(core): serialize screen-file test with the TMPDIR-mutating test

screen_file_contents_and_write occasionally failed with NotFound when
run in parallel with screen_temp_file_keeps_non_utf8_path_bytes: that
test sets TMPDIR to a short-lived directory via temp_env, and
std::env::temp_dir() evaluated outside temp_env's lock could resolve
into it. The temp-file section now runs under temp_env's lock with
TMPDIR unset. 25 consecutive runs of the core lib tests pass.

crates/ghostty-core/src/surface/binding.rs

@@ -1286,10 +1286,19 @@ mod tests {
                 .is_none()
         );
 
-        let path =
+        // Another test in this binary changes TMPDIR via temp_env, so the
-            write_screen_temp_file(&s, WriteScreenLoc::Screen, WriteScreenFormat::Plain).unwrap();
+        // whole temp-file section runs under temp_env's lock with TMPDIR
+        // unset (std::env::temp_dir() then cannot point into that test's
+        // short-lived directory).
+        temp_env::with_var_unset("TMPDIR", || {
+            let own_tmp = std::env::temp_dir()
+                .join(format!("ghostty-screen-file-test-{}", std::process::id()));
+            std::fs::create_dir_all(&own_tmp).unwrap();
+            let path = temp_env::with_var("TMPDIR", Some(&own_tmp), || {
+                write_screen_temp_file(&s, WriteScreenLoc::Screen, WriteScreenFormat::Plain)
+                    .unwrap()
+            });
             assert!(path.ends_with("screen.txt"));
-        let path = path.to_string_lossy().into_owned(); // test-only: ASCII temp path
             let read = std::fs::read_to_string(&path).unwrap();
             assert_eq!(read, s);
             #[cfg(unix)]
@@ -1299,7 +1308,8 @@ mod tests {
                 );
                 assert_eq!(mode & 0o777, 0o600);
             }
-        let _ = std::fs::remove_dir_all(std::path::Path::new(&path).parent().unwrap());
+            let _ = std::fs::remove_dir_all(&own_tmp);
+        });
     }
 
     #[cfg(unix)]