build-aux/run-test.sh.in: Write the test log to stderr

With the automake parallel test harness the test's stdout and stderr both
go only to its log file, which run-test.sh then printed to stdout. CI scans
only "make check" stderr for UBSan "runtime error:" lines, and UBSan errors
don't fail the test, so they were silently ignored.

Broken by 9470688ee1

build-aux/run-test.sh.in

@@ -1,6 +1,6 @@
 #!/bin/sh
 
-## serial 3
+## serial 4
 
 set -eu
 
@@ -24,7 +24,9 @@ function cat_log_file() {
   while [ $# -gt 0 ]; do
     case "$1" in
       --log-file)
-	cat $2
+	# The log contains also the test's stderr (e.g. UBSan errors),
+	# so keep it visible in stderr.
+	cat $2 >&2
 	break
 	;;
       *)