Give directions on attaching gdb to run-test target

Attaching to run-test on target requires the use of the gdbclient.py
program and can be difficult to remember. This makes run-test print a
reminder when you run --gdb on target.

Test: none.
Change-Id: I685c4ed5b5ca19fced183e57efa04b2dca057685
diff --git a/test/etc/run-test-jar b/test/etc/run-test-jar
index bd58ae3..4ebe1b8 100755
--- a/test/etc/run-test-jar
+++ b/test/etc/run-test-jar
@@ -988,6 +988,7 @@
 
     if [ "$USE_GDB" = "y" ]; then
       # When running under gdb, we cannot do piping and grepping...
+      echo "Run 'gdbclient.py -p <pid printed below>' to debug."
       $cmdline "$@"
     else
       if [ "$TIME_OUT" != "gdb" ]; then