yocto/system-tests: Run bats with extra debug messages

Introduce the BATS_EXTRA_OPTIONS variable to be passed to bats with the
following default value:

BATS_EXTRA_OPTIONS ?= "\
--verbose-run \
--trace \
--timing \
--print-output-on-failure \
--show-output-of-passing-tests"

Issue-Id: SCM-5314
Signed-off-by: Diego Sueiro <diego.sueiro@arm.com>
Change-Id: Ie34d5e953cab4cf4441e512a98a76b57003f3383