Unit tests: Fix pin validation failure
Unit tests: Fix pin validation failure

The test script pinvalidate.py requires the following which are
missing in the unit test stub PinName.h:
* A comment "MBED TARGET LIST"
* `CONSOLE_TX` and `CONSOLE_RX` in the `PinName` enum

This commit adds them.

mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed