[SUREFIRE-1542] Fractional per-core thread count

**[Chris Hennick](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=pr0methean)** opened **[SUREFIRE-1542](https://issues.apache.org/jira/browse/SUREFIRE-1542?redirect=false)** and commented

I have a lot of tests that each use 2 threads at once, and timeouts become a problem when I try to run 1 test thread per core (because that amounts to 2 actual active threads per core). Why not make the thread count floating-point, so that I could fix this by specifying 0.5 threads per core?

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No further details from [SUREFIRE-1542](https://issues.apache.org/jira/browse/SUREFIRE-1542?redirect=false)

Start with the linked SUREFIRE-1542 issue and trace how the existing per-core thread-count setting is parsed and applied. Determine the expected behavior for a value such as 0.5, then add coverage showing that fractional values produce the intended effective concurrency and that existing values remain supported.

Written by the indexing model from the issue text.