The help is a bit of as wall of text and confusing, but the summary looks like so:

$ ./pants test.junit --help
test.junit options:
...
cache.test.junit options:
...
jvm.test.junit options: 
...
--jvm-test-junit-options=<option>... (--jvm-test-junit-options=<option>...) ..., --jvm-test-junit-options="[<option>..., <option>..., ...]", --jvm-test-junit-options="+[<option>..., <option>..., ...]" (default: ['-Djava.awt.headless=true','-Xmx1g','-XX:MaxPermSize=256m'])
    Run with these extra JVM options.
...