sencha: set path to testbench firmware

Depending on the default path in cheriot.resc was too fragile.

Bypass-Presubmit-Reason: no CI tests for sencha

Change-Id: I8f45ea45205e1d426c711a1af2b013481ead2d5a
diff --git a/platforms/sencha/ b/platforms/sencha/
index 309e1a7..297dada 100644
--- a/platforms/sencha/
+++ b/platforms/sencha/
@@ -19,7 +19,9 @@
 ## Runs the cheriot-rtos testbench in renode
 cheriot-testbench: renode cheriot_sim cheriot-test
-	$(RENODE_CMD) -e "i @${CHERIOT_RESC_RELEASE}; start"
+	$(RENODE_CMD) -e "\
+    \$$bin =@${CHERIOT_TEST}; \
+    i @${CHERIOT_RESC_RELEASE}; start"
 ## Generates the cheriot-rtos test-suite firmware