check-sr-results: rework overlay conditions

Now that we want to determine the SystemReady version of results based on
more than the EBBR sequence file, change the `ebbr_seq_files' condition to
more generic `when-any' and `when-all' conditions.
Only minimal functional change in the messages is expected.
We test a bit in the unit test.

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>