ctest: Allow passing -j without value to choose a contextual default
Under job server integration, added by !9021 (merged), use a very high default so that parallelism is effectively limited only by available job server tokens.
Otherwise, choose a default limit based on the number of processors.
Also allow passing 0
to specify unbounded parallelism.
Fixes: #25739 (closed)
Backport: release
Edited by Brad King