tests: virtio-mem: Use the shared fillmem helper

check_memory_usable() runs fillmem to completion and greps its status
file for success, which is what make_guest_dirty_memory() now does for
every caller. Use the shared helper and keep the 30 second bound.

The helper also lowers sshd's OOM score before starting fillmem, and
polls the status file every half second instead of blocking on the SSH
command. The check only asserts that the allocation succeeded, so
neither changes what it verifies.

Signed-off-by: James Curtis <jxcurtis@amazon.co.uk>