Restrict number of testrunner jobs to be equal to # cpus.

If not explicitly provided, testrunner set the concurrency to the
number of available cpus.

Bug: 65822340
Test: ./art/test/testrunner/run_build_test_target.py art-forcecopy -j100
Change-Id: I9e1473e0739289e5307ae618f0fe6fe5debf720c