zephyr: Test uart_buffer_full
zephyr: Test uart_buffer_full

Validate uart_buffer_full

BUG=None
BRANCH=NONE
TEST=./twister -T zephyr/test

Signed-off-by: Al Semjonovs <c3a2ecb550cad1e7d5a943ca6241377f7b43ffc4@google.com>
Change-Id: I148dee0af88b3b030350cb438586df6040d66234
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3984428
Reviewed-by: Aaron Massey <79529a820df71f45023239a3663736629646cc34@google.com>
Commit-Queue: Aaron Massey <79529a820df71f45023239a3663736629646cc34@google.com>
Code-Coverage: Zoss <345cf107e82c70959d46f59d98bddc98ce458aff@prod.google.com>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec