virtio-devices: vsock: fix two clippy warnings

The mem_size field is not needed in TestContext. Drop it.

Make sure guest_evvq is read once. Clippy cannot figured out that it was
used.

While at it, add an extra assert for the spurious rxvq event test, too.

Signed-off-by: Wei Liu <liuwe@microsoft.com>

virtio-devices/src/vsock/device.rs

@@ -782,6 +782,7 @@ mod tests {
             let mut epoll_helper =
                 EpollHelper::new(&ctx.handler.kill_evt, &ctx.handler.pause_evt).unwrap();
 
+            assert_eq!(ctx.guest_rxvq.used.idx.get(), 0);
             assert!(
                 ctx.handler.handle_event(&mut epoll_helper, &event).is_err(),
                 "handle_event() should have failed"
@@ -802,6 +803,7 @@ mod tests {
             let mut epoll_helper =
                 EpollHelper::new(&ctx.handler.kill_evt, &ctx.handler.pause_evt).unwrap();
 
+            assert_eq!(ctx.guest_evvq.used.idx.get(), 0);
             assert!(
                 ctx.handler.handle_event(&mut epoll_helper, &event).is_err(),
                 "handle_event() should have failed"