Allow overhead tests to specify a warm-up period

[breedx-splk]

In #3627, it was noted that we may wish to have some tests run after a warm-up phase. That is, we don't want to actually start the measurements (with JFR) until after some warm-up cycle so that the jit compilation has had a chance to do its thing and the app has reached more of a steady-state.

As noted in #3627, there are pros and cons of doing this. On one hand, will miss the overhead introduced by the agent during startup, but for many users (especially those with long-running enterprise JVM applications) they may only care about steady-state overhead.

We should modify the overhead test configuration to allow for an optional warm-up cycle, specified in seconds, during which a moderate amount of traffic should be generated. Only after the startup has completed should the relevant measurements begin. With JFR, this can be accomplished with the delay parameter (dynamically configured on the commandline via system props).