Integrate QSerialPort auto-tests to CI

To minimize an issues (like QTBUG-78086) it would be good to have an auto-tests integrated into CI.

For this purpose need to install on the CI the following drivers:

• Windows (32/64 bit) - com0com
• Linux - tty0tty (can be available in repos on some Linux distros)

That drivers creates a pair of virtual serial ports (e.g.

COM3 <-> COM4

on Windows, and

/dev/tnt0 <-> /dev/tnt1

on Linux).

The QSP's auto-tests provides two env variables: QTEST_SERIALPORT_SENDER and QTEST_SERIALPORT_RECEIVER. So, it is enough to set this variables before the tests launching, e.g.

• on Windows:

set QTEST_SERIALPORT_SENDER=COM3
set QTEST_SERIALPORT_RECEIVER=COM4

• on Linux:

export QTEST_SERIALPORT_SENDER=/dev/tnt0
export QTEST_SERIALPORT_RECEIVER=/dev/tnt1