sim:tests: tweak stress test

Wait for the prompt after issuing an "mdebug" command to workaround
parsing concurrent output (changed with rust-templates).

Change-Id: Id1b7f7e6d3d14ccde6a2625c05a2e01633b33192

shodan_stress.robot

diff --git a/shodan_stress.robot b/shodan_stress.robot
index 5778ab3..b2fbefb 100644
--- a/shodan_stress.robot
+++ b/shodan_stress.robot
@@ -99,4 +99,5 @@
       END
 
       Write Line to Uart          mdebug
+      Wait For Prompt On Uart     ${PROMPT}
     END