bsc: Fix dangling subscriber talloc ctx in f_TC_paging_Nreq

First, page only on BTS0, not all the BTS. This simplifies the test and
also makes the subscriber be refcounted only on BTS0.
Then, drop the OML connection after receiving all the expected paging
requests; it will make the BSC flush the paging queue and hence avoid
TTCN3 checks erroring due to dangling subscriber talloc contexts.

Change-Id: Ibbd9168f3ac2c1cd4f4577644d75499d49c2823f
1 file changed