Adds physics tests
Adds physics tests

Physics tests are similar to rattests in rat. These should be used to check
physics values returned by funtions, which is not something you usually want to
check with a unittest.

jwaterfield/echidna,mjmottram/echidna,drjeannewilson/echidna,arushanova/echidna,snoplusuk/echidna,ashleyrback/echidna