Testconfig: Enabling QtSerialPort tests in CI

Each win 7 is now having null-modem emulator with COM5 and COM6
ports. CI will export required env variables for all CI jobs, but
it has no effect except when testing QtSerialPort or Qt5 with
QtSerialPort.

Task-number: QTQAINFRA-682
Change-Id: I5671e859ef282467db225bccaa4a2ff6ce379a99
Reviewed-by: Laszlo Papp <lpapp@kde.org>
Reviewed-by: Tony Sarajärvi <tony.sarajarvi@digia.com>
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>