Stabilize bgsave test that sometimes fails with valgrind (#7559)

on ci.redis.io the test fails a lot, reporting that bgsave didn't end.
increaseing the timeout we wait for that bgsave to get aborted.
in addition to that, i also verify that it indeed got aborted by
checking that the save counter wasn't reset.

add another test to verify that a successful bgsave indeed resets the
change counter.