cli: Ensure "plan exec" remains compatible with old containers

Commit ac88ccb ("cli: Remove RESULTCLASS summary field") removed the
RESULTCLASS.summary field, and as a result this was removed from the
results.csv file passed back from the container to the test runner. This
was a breaking change and was intentional; The plan was to rebuild the
containers.

But it turns out we can't easily rebuild the speedometer container
because the version of Chromium that the old version was using is no
longer available. Changing the browser version will change the benchmark
performance so a new container needs to be properly versioned.

So let's just allow the old container to work with the new code. We
allow "summary" but ignore it.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>