Proof tests: the user-dictionary file test waits for the dictionary tests

user_dictionary_persists_through_a_file saves the whole user dictionary to
a file and loads it back. Run beside
user_dictionary_and_ignore_all_match_both_normalization_forms, it could
catch a word that test had added and bring it back after the test removed
it, so the other test's last check failed now and then (once in about two
dozen full runs here). It now holds DICTIONARY_TESTS, as the other test
does.

crates/proof/src/lib.rs

@@ -1059,7 +1059,9 @@ mod tests {
 
     use super::*;
 
-    /// Held by tests that swap or rely on the shared Spanish dictionary, so a folder reset can't land mid-test.
+    /// Held by tests that swap or rely on the shared Spanish dictionary, so a folder reset can't land
+    /// mid-test, and by those that save the whole user dictionary and load it back, which would
+    /// bring back a word another test just removed.
     static DICTIONARY_TESTS: std::sync::Mutex<()> = std::sync::Mutex::new(());
 
     /// #1063: the dictionaries folder is searched before the system locations; the dictionaries
@@ -1579,6 +1581,7 @@ mod tests {
     /// #40: the user dictionary survives a save and a fresh load.
     #[test]
     fn user_dictionary_persists_through_a_file() {
+        let _guard = DICTIONARY_TESTS.lock().unwrap_or_else(std::sync::PoisonError::into_inner);
         let dir = std::env::temp_dir().join(format!("wc-dict-{}-{}", std::process::id(), "persist"));
         let _ = std::fs::remove_dir_all(&dir);
         // The folder is created on the first save.