Increase the concurrency when running UTs in pre commit job

The default forkCount is 0.25C. We used to run on a 16 cores machine so the concurrency is 4, but now our build nodes are all 8 cores, so the concurrency is 2. The build time is way more slower than before.

Since we only have 2 slots on the build node, I think it is OK to increase the forkCount to 0.5C. But I'm not sure whether we should change the default value in pom, or we just set it in the pre commit job.