tests/sys_arduino: fix test synchronization issues

The test currently doesn't wait for synchronization of the serial
terminal the second time it is setup with `Serial.begin`. This results
in the test commands being sent before the terminal is ready which
causes the test to fail. To fix this add an expect_exact on "Hello
arduino". (NOTE: this is different form `interactive_sync` which
synchronizes the start of the test, here we want `uart` to properly
re-init)

tests/sys_arduino/tests/01-run.py

@@ -11,6 +11,9 @@ from testrunner import run
 
 
 def testfunc(child):
+    # Wait for serial.begin() to complete
+    child.expect_exact("Hello Arduino!")
+
     # 1 Basic read+write test on serial with error command
     child.sendline("wrang")
     child.expect_exact("UNK")