tests/stats: enable logging in test output

Move test output from stdout to stderr and enable logging to stderr.
This is in preparation for the next patch, which will add a new log
message when osmo_stat_item_get_next() skips a value.

Related: SYS#4877
Change-Id: Ie0eaec2f93ac6859397a6bfca45039fdcc27cb9e
diff --git a/tests/testsuite.at b/tests/testsuite.at
index d2a10dd..cb84229 100644
--- a/tests/testsuite.at
+++ b/tests/testsuite.at
@@ -249,7 +249,8 @@
 AT_SETUP([stats])
 AT_KEYWORDS([stats])
 cat $abs_srcdir/stats/stats_test.ok > expout
-AT_CHECK([$abs_top_builddir/tests/stats/stats_test], [0], [expout], [ignore])
+cat $abs_srcdir/stats/stats_test.err > experr
+AT_CHECK([$abs_top_builddir/tests/stats/stats_test], [0], [expout], [experr])
 AT_CLEANUP
 
 AT_SETUP([write_queue])