test-runner: fix an uninitialized-memory error

Clippy is failing with this error `memory_map`:

    error: calling `set_len()` immediately after reserving a buffer creates uninitialized values

Since the code here is just test code where performance isn't a concern,
fully initialize the vec with zeros and drop the unsafe code.

Author: nicholasbishop

uefi-test-runner/src/boot/memory.rs

@@ -91,11 +91,7 @@ fn memory_map(bt: &BootServices) {
     let buf_sz = map_sz + 8 * mem::size_of::<MemoryDescriptor>();
 
     // We will use vectors for convencience.
-    let mut buffer = Vec::with_capacity(buf_sz);
-
-    unsafe {
-        buffer.set_len(buf_sz);
-    }
+    let mut buffer = vec![0_u8; buf_sz];
 
     let (_key, desc_iter) = bt
         .memory_map(&mut buffer)