test: ut can't process test command:

check ut reset done condition is not correct. If bootup pattern already
exist in cache, we will return without waiting for DUT bootup.

tools/unit-test-app/unit_test.py

@@ -141,6 +141,8 @@ def replace_app_bin(dut, name, new_app_bin):
 
 
 def reset_dut(dut):
+    # We do flush before test, in case we already have bootup pattern in data cache
+    dut.write("", flush=True)
     dut.reset()
     # esptool ``run`` cmd takes quite long time.
     # before reset finish, serial port is closed. therefore DUT could already bootup before serial port opened.