Exception handling for lines > 254 bytes

[aldenhart]

The serial input will appear to receive lines of > 254 bytes but does not process them correctly. See this test in tg_pytest:

/data/g2-0.99/004-specials/serial/serial.json test:
"Input is 256 characters in length, including terminating LF"

This test will appear to function. The test following will fail with the residual "56" (from the ____256) appearing as the input.