uefi-raw: add test for MemoryDescriptor to catch Miri problems

The underlying issue is that if someone wants to cast a slice of descriptors to a
byte slice, the uninitialized **implicit** padding bytes are UB. Miri complains
about that.

uefi-raw/src/table/boot.rs

@@ -486,3 +486,34 @@ pub enum Tpl: usize => {
 /// Note that this is not necessarily the processor's page size. The UEFI page
 /// size is always 4 KiB.
 pub const PAGE_SIZE: usize = 4096;
+
+#[cfg(test)]
+mod tests {
+    use core::{mem, slice};
+    use super::*;
+
+    // This tests succeeds if Miri doesn't complain about uninitialized memory.
+    #[test]
+    fn memory_descriptor_trivial_serialization() {
+        let descs = [
+            MemoryDescriptor {
+                ty: MemoryType::CONVENTIONAL,
+                phys_start: 0x1000,
+                virt_start: 0x1000,
+                page_count: 1,
+                att: Default::default(),
+            },
+        ];
+
+
+        let raw_bytes: &[u8] = {
+            let ptr = descs.as_ptr().cast::<u8>();
+            let len = mem::size_of_val(&descs);
+            unsafe { slice::from_raw_parts(ptr.cast(), len) }
+        };
+
+        // Test succeeds if Miri doesn't complain about initialized memory
+        // (implicit padding fields).
+        dbg!(&raw_bytes);
+    }
+}