test_solvers: opt_workers does not default to correct value

[michaelcadilhac]

This initializes opt_workers to 0:

https://github.com/trolando/oink/blame/257d0d088361204449c9bb204b457c2042e26cae/test/test_solvers.cpp#L53

The documentation reads:

  -w, --workers arg    Number of workers for parallel algorithms, or -1 for 
                       sequential, 0 for autodetect (default: -1)