Add a test for checking if tests are run single-threadedly.

tests/integration_tests.rs

@@ -68,6 +68,17 @@ mod tests {
         rust_i18n::i18n!(fallback = "foo");
     }
 
+    #[test]
+    fn check_test_environment() {
+        assert_eq!(
+            std::env::var("RUST_TEST_THREADS").unwrap_or_else(|_| "0".to_string()),
+            "1",
+            "The tests assume that they are run in single-threaded environment because \
+            setting the locale is a global state. If using cargo version prior to 1.56, \
+            You should set RUST_TEST_THREADS instead of running with --test-threads."
+        );
+    }
+
     #[test]
     fn test_load() {
         assert!(load_locales("./tests/locales", |_| false)