gitlab: add optional job to run flaky avocado tests

gitlab: add optional job to run flaky avocado tests

One problem with flaky tests is they often only fail under CI
conditions which makes it hard to debug. We add an optional allow_fail
job so developers can trigger the only the flaky tests in the CI
environment if they are debugging.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20231201093633.2551497-8-alex.bennee@linaro.org>