allow deref of null ptr in test

tests/compile-fail/null_pointer_deref.rs

@@ -1,3 +1,4 @@
+#[allow(deref_nullptr)]
 fn main() {
     let x: i32 = unsafe { *std::ptr::null() }; //~ ERROR inbounds test failed: 0x0 is not a valid pointer
     panic!("this should never print: {}", x);

tests/compile-fail/null_pointer_deref_zst.rs

@@ -1,6 +1,7 @@
 // Some optimizations remove ZST accesses, thus masking this UB.
 // compile-flags: -Zmir-opt-level=0
 
+#[allow(deref_nullptr)]
 fn main() {
     let x: () = unsafe { *std::ptr::null() }; //~ ERROR memory access failed: 0x0 is not a valid pointer
     panic!("this should never print: {:?}", x);

tests/compile-fail/null_pointer_write.rs

@@ -1,3 +1,4 @@
+#[allow(deref_nullptr)]
 fn main() {
     unsafe { *std::ptr::null_mut() = 0i32 }; //~ ERROR inbounds test failed: 0x0 is not a valid pointer
 }

tests/compile-fail/null_pointer_write_zst.rs

@@ -1,6 +1,7 @@
 // Some optimizations remove ZST accesses, thus masking this UB.
 // compile-flags: -Zmir-opt-level=0
 
+#[allow(deref_nullptr)]
 fn main() {
     // Not using the () type here, as writes of that type do not even have MIR generated.
     // Also not assigning directly as that's array initialization, not assignment.