Servo V3: Update flex tests for V3.

This change ensures that the flex tests script will work with Servo
V3 with the 50-40 pin and the 50-50 pin flexes. Adds a new argument
--v3 to specify you are using servo v3 and checks jtag, spi, uart
and basic gpios.

Also flips the uart_en lines as they were reversed.

BUG=chromium:271524
TEST=local setup.

Change-Id: Ia460a523a87af40d30ed6163e1909dfbb89fc1b3
Reviewed-on: https://chromium-review.googlesource.com/66459
Reviewed-by: Todd Broch <tbroch@chromium.org>
Commit-Queue: Simran Basi <sbasi@chromium.org>
Tested-by: Simran Basi <sbasi@chromium.org>
6 files changed