Add a build property which passes extra JVM arguments in to the testsuite's server process

Right now the only way to easily pass extra arguments in to the test server JVMs (e.g. for profiling, or configuration) seems to be to use the server.jvm.args property. However using this property directly erases -Xmx and a bunch of other important things.

There should be a property which is empty by default that gets merged into the final argument list for server.jvm.args, like extra.server.jvm.args or something.