tests: add java reclaim test

This is a test in which two threads compete for resources. One of them will
(hopefully) trigger memory allocations that are served by the heap while the other
will stress the filesystem through reads and/or writes (no mappings).

This is designed to test how well the balloon code works together with the ARC
reclaimer.

There are three main goals I expect OSv to achieve when running this test:

1) When there is no filesystem activity, the balloon should never trigger, and
the ARC cache should be reduced to its minimum
2) When there is no java activity, we should balloon as much as we can, leaving
the memory available to the filesystem (this one is trickier because the IO code
is itself a java application - on purpose - so we eventually have to stop)
3) When both are happening in tandem, the system should stabilize in reasonable
values and not spend useless cycles switching memory back and forth.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>