add PhysicActionTestCase, SetMassActionTest

added BaseClass for PhysicActionTests and setMassActionTest
modified setMassBehavior - now zero mass is a valid input
deleteTestProjects, SetMassActionTest structure
deleteTestProjects -> PhysicActionTestCase.tearDown()
setUp() -> TestUtils.deleteTestProjects()
Merge pull request #978 from Catrobat/physics_BasePhysicTestClass

add PhysicActionTestCase, SetMassActionTest