block: qcow: Test rejection when backing file overlaps header

Cover the case where backing_file_offset points inside the fixed
header fields, which the new header overlap check must reject.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>

block/src/formats/qcow/internal/mod.rs

@@ -2747,6 +2747,12 @@ mod unit_tests {
         ));
     }
 
+    #[test]
+    fn backing_file_offset_inside_header() {
+        let err = read_header_with_patched_backing(64, 16).unwrap_err();
+        assert!(matches!(err, Error::BackingFileOverlapsHeader(_, _, _)));
+    }
+
     #[test]
     fn backing_file_fits_at_cluster_end() {
         let cluster_size = 1u64 << DEFAULT_CLUSTER_BITS;