bts: Account for elapsed time in paging timer

Previous implementation always waited for "interval" time until sending
next paging cmd, and didn't finish the test until all expected paging
cmds were sent. As a result, each time it triggered it accumulated some
delay which could go from 2 seconds to 12 seconds depending on machine
load.
As a consequence, the expected number of paging cmd messages to be sent
in 20 seconds was being sent in 22-32 seconds, hence changing the load
on osmo-bts and as a result changing the test results.

Low threshold needs to be adapted since now they are sent in exactly 20
seconds max and the load handled by osmo-bts is bigger.

Fixes: OS#3025
Change-Id: I9651136d6810420e0a4d887bfb11c913a24f0457
1 file changed