5.0.0-rc3: Testing signal generation (OUTPUT vs. INPUT_PULLUP)

against 3rd party hardware, ESP8266 internal UART,
Wemos D1 mini USB UART.
Removed ALT_DIGITAL_WRITE.
Commited tests Loopback and Repeater in default
configuration to run two ESP8266 internal UARTs
connected - max. reliable bitrate is only 2400bps in
tests.