Add test for uninit raw ptrs

Author: camelid

tests/compile-fail/uninit_raw_ptr.rs

@@ -0,0 +1,4 @@
+fn main() {
+    let _val = unsafe { std::mem::MaybeUninit::<*const u8>::uninit().assume_init() };
+    //~^ ERROR type validation failed at .value: encountered uninitialized raw pointer
+}