qa/lsan.supp: suppress seastore reactor-teardown leaks

unittest-seastore runs the seastar reactor on a separate thread
(SeastarRunner) and stops it at exit without draining its pending
tasks, so a few cached extents those tasks still held are leaked at
shutdown. Suppress them by the three seastore subsystems they come from.

Signed-off-by: Sun Yuechi <sunyuechi@iscas.ac.cn>