Disable most UB checks by default

Author: cbeuw

difftest/src/main.rs

@@ -38,9 +38,9 @@ fn main() -> ExitCode {
     }
 
     if let Ok(miri_dir) = settings.get_string("miri_dir") {
-        let check_ub = !settings
+        let check_ub = settings
             .get_string("miri_check_ub")
-            .is_ok_and(|config| config == "false" || config == "0");
+            .is_ok_and(|config| config == "true" || config == "1");
         let miri = Miri::from_repo(miri_dir, check_ub);
         match miri {
             Ok(miri) if check_ub => backends.insert("miri-checked", Box::new(miri)),