Under job server integration, added by commit 80fe56c4 (ctest: Add
support for running under a make job server on POSIX systems,
2023-11-15, v3.29.0-rc1~324^2), 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