Use ASSERT_TIMELY in tests where possible

[wezrule]

ASSERT_TIMELY provides a more concise syntactic way to express:

system.deadline_set (10s);
while (!condition)
{
	ASSERT_NO_ERROR (system.poll ());
}

as
ASSERT_TIMELY (10s, condition);

This cannot be used in more complex examples such as when there are other contents of the body. For that we do have system::poll_until_true, but that has not been used here.

Also modified some places which used a read db transaction variable in each iteration inside the loop body and doing a comparison there. This can be changed to create a temporary transaction as an argument, allowing the use of ASSERT_TIMELY with it.