test: When kdc fails to start, show the log

This is an attempt to find out why the GitHub Actions OS X build is
failing.

tests/gss/check-basic.in

@@ -94,7 +94,7 @@ echo "Doing database check"
 ${kadmin} check ${R} || exit 1
 
 echo Starting kdc
-${kdc} --testing --detach || { echo "kdc failed to start"; exit 1; }
+${kdc} --testing --detach || { echo "kdc failed to start"; cat messages.log; exit 1; }
 kdcpid=`getpid kdc`
 
 trap "kill ${kdcpid}; echo signal killing kdc; exit 1;" EXIT