test-send-recv: Fix occasional timeouts

While waiting for write thread to finish sending, keep also receiving so
that any STUN messages from the peer still get processed.
6 jobs for <span class="ref-name">fix-timeouts</span> in 9 minutes and 17 seconds (queued for 93 minutes and 20 seconds)
Status Job ID Name Coverage
  Build
passed #154496
build autotools

00:01:50

passed #154497
build meson

00:00:51

 
  Test
passed #154499
distcheck autotools

00:02:44

passed #154501
doc-and-install meson

00:01:10

passed #154498
test autotools

00:01:52

passed #154500
allowed to fail
test meson

00:00:48