In theory, you have 2 ways for running tests in parallel on Maven. One is using -T command line option (that basically builds multiple modules at the same time), the other is configuring the surefire plugin for running multiple tests at the same time.
Unfortunately, both approaches won't work with RESTEasy, as the testsuite has been written in the past with the assumption of a single test is running at a time. So you'd basically get a lot of failures.
Allowing for a better use of CPU resources and hence speeding up the build is in my TODO list, but so far we've had to deal with more urgent tasks.