SL-793: Fix lllogin_test.cpp for new LLCoros implementation.

Delete the test for SRV timeout: lllogin no longer issues an SRV query. That
test only confuses the test program without exercising any useful paths in
production code.

As with other tests dating from the previous LLCoros implementation, we need a
few llcoro::suspend() calls sprinkled in so that a fiber marked ready -- by
fulfilling the future for which it is waiting -- gets a chance to run.

Clear LLEventPumps between test functions.