Tell MTR to not save data-dirs and core files

Currently there's no way of running the test suite without generating some cruft files like the var datadirs and core dumps. This turns into a problem when running in memory with --mem and /dev/shm fills up pretty quickly with no obvious way of cleaning up; besides manual removal from the shared memory directory.

Currently --max-save-datadir and max-save-core only accept a positive numerical value or 0 which means unlimited.

Problem encountered on normal Buildbot workers running bare bone builds that are not discardable like KVM machines or Docker images.