tests/docker: make docker engine choice entirely configure driven

tests/docker: make docker engine choice entirely configure driven

Since 0b1a649047 (tests/docker: use direct RUNC call to build
containers) we ended up with the potential for the remaining docker.py
script calls to deviate from the direct RUNC calls. Fix this by
dropping the use of ENGINE in the makefile and rely entirely on what
we detect at configure time.

We also tweak the RUNC detection so podman users can still run things
from the source tree.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20231009164104.369749-6-alex.bennee@linaro.org>