disable validation for some tests that need further investigation

tests/run-pass-fullmir/send-is-not-static-par-for.rs

@@ -8,6 +8,9 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// FIXME: Still investigating whether there is UB here
+// compile-flags: -Zmiri-disable-validation
+
 use std::sync::Mutex;
 
 fn par_for<I, F>(iter: I, f: F)

tests/run-pass-fullmir/u128.rs

@@ -8,6 +8,9 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// FIXME: Still investigating whether there is UB here
+// compile-flags: -Zmiri-disable-validation
+
 fn b<T>(t: T) -> T { t }
 
 fn main() {

tests/run-pass-fullmir/vecdeque.rs

@@ -1,3 +1,6 @@
+// FIXME: Still investigating whether there is UB here
+// compile-flags: -Zmiri-disable-validation
+
 use std::collections::VecDeque;
 
 fn main() {