lazy check for serial port opening

[Sneezry]

Related issue: #530 TestingOpen output on the serial monitor

This is a workaround for serial port opening test. To avoid disturbing user, I remove auto sending TestingOpen after serial port opened.

This fix may cause a small issue:

When we get serial port open event, we print a message says Opened the serial port. However, we cannot verify if the serial port open successfully until the user send some text by using sendMessage (related issue logged here: serialport/node-serialport#795).

Thus, the user may see two contradictory messages after send some text in a chance, one says serial port opened, one says open serial port opened failed.

[Sneezry]

If the user just read data from serial port, but not write data, he/she will see serial port opened successfully but not receive any data.