I think you need to clarify a bit: You seem want to run multiple scripts simultaneously, each of which may (or may not) modify a single list of tests to be run. How many tests? What sort of load does each put on your system(s)? Each test against the same program or data source or each against multiple files or ???

This has huge potential for mischief, even if you can make it work (or, "work reliably"), and seems unlikely to be faster than using forks or threads for the various tests to be executed.

In any case (and this is merely a guess based on insufficient information), any bottlenecks are apt to have their roots in disk access and RAM limits.

And, for the record, please see such topics as How do I post a question effectively? and Markup in the Monastery.