turn: Allow alternate server error with no valid credentials

coturn doesn't authenticate us before sending us to an alternate server.
8 jobs for 66-fix-alternate-server-with-coturn in 12 minutes and 56 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #88136
build autotools

00:00:52

passed #88137
build meson

00:08:44

 
  Test
passed #88139
distcheck autotools

00:02:03

passed #88162
doc-and-install meson

00:01:52

passed #88138
test autotools

00:01:29

failed #88140
allowed to fail
test meson

00:02:19

failed #88155
doc-and-install meson

00:00:13

failed #88141
doc-and-install meson

00:02:18

 
Name Stage Failure
failed
test meson Test
--- command ---
BUILT_WITH_MESON='1' /builds/ocrete/libnice/build/tests/nice-test-new-dribble
-------

Full log written to /builds/ocrete/libnice/build/meson-logs/testlog.txt
Uploading artifacts...
build/meson-logs/: found 4 matching files
Uploading artifacts to coordinator... ok id=88140 responseStatus=201 Created token=HEkqe7oM
ERROR: Job failed: exit code 1