Refactor run_tests to increase parallelism.
Refactor run_tests to increase parallelism.

Rather than relying on parallel make to build individual configurations
one at a time, use xargs to build multiple configurations in parallel.
This allows the configure scripts to run in parallel.  On a 14-core
system (28 hyperthreads), this increases average CPU utilization from
~20% to ~90%.

wqfish/jemalloc,wqfish/jemalloc,wqfish/jemalloc,wqfish/jemalloc,wqfish/jemalloc