give appropriate memory settings to ITs

On machines with limited memory, the default heap may not be enough to get
the correct BDB profile - passing a max heap size will ensure it will be
available if possible. We must configure this in the POM, as on the command
line it will clash with the prepare-agent goal of the JaCoCo plugin (see
http://www.eclemma.org/jacoco/trunk/doc/prepare-agent-mojo.html for more
information). By setting the right setting in the properties, jacoco will
pick it up and prepend to it when constructing the agent, but it will still
be used when jacoco hasn't been run (i.e. during sonar:sonar)