tests/shell: wait for prompt at beginning

Previously, the test would start sending the first test commands right
at the beginning. This fails on boards whose UART is not ready at that
point.

This PR makes the test script explicitly wait for the prompt to be
ready.

tests/shell/tests/01-run.py

@@ -40,6 +40,7 @@ CONTROL_C = DLE+'\x03'
 CONTROL_D = DLE+'\x04'
 
 CMDS = (
+    (CONTROL_C, ('>')),
     ('start_test', ('[TEST_START]')),
     ('end_test', ('[TEST_END]')),
     ('\n', ('>')),