convert all shared pointers in tests to stack-allocated instances

[gchenfc]

This issue was imported from github.gatech.edu.
The original issue was opened by @smccormick32 on 2020-07-08T15:25:16Z

From review of PR https://github.gatech.edu/borglab/GTDynamics/pull/47, line 65 in testPrismaticJoint.cpp and line 55 in testScrewJoint.cpp. (This is likely also present in testRevoluteJoint.cpp, and may show up in other tests as well.)

Here is an example of what that looks like currently, where j1 is the shared pointer to be converted in this case:

EXPECT(j1->getJointParameters().effort_type == Joint::JointEffortType::Actuated);

[varunagrawal]

List of files this needs to be updated in:

1. testSdf.cpp
2. testHelicalJoint.cpp
3. testPrismaticJoint.cpp