Convert all unit tests into pytest format

[tomsilver]

This is an addendum to #82 . I want to take stock of the current status of the existing unit tests (after converting them into the pytest format).

Test	Pass	Comments
test_articulation_ik.py	- [x]	Passes in 18 seconds
test_articulation_root.py	- [x]	Passes in 8 seconds
test_camera_image.py	- [x]	Passes in 13 seconds
test_cloth_attach.py	- [ ]	Crashes, possibly due to missing files
test_custom_message.py	- [x]	Passes in 1.5 seconds
test_debug.py	- [x]	Passes in 50 seconds
test_digit.py	- [x]	Passes in 1.5 seconds
test_gelslim.py	- [x]	Passes in 4 seconds
test_gelslim.py	- [x]	Passes in 4 seconds
test_grasp_pose.py	- [ ]	Crashes due to missing files
test_grasp_sim.py	- [ ]	Crashes due to missing files
test_heat_map.py	- [ ]	Requires user interaction
test_humanbody_ik.py	- [x]	Passes in 22 seconds
test_image_stream.py	- [x]	Passes in 13 seconds
test_label.py	- [ ]	Crashes with KeyError: 'rgb'
test_light.py	- [x]	Passes in 9 seconds
test_load_mesh.py	- [ ]	Doesn't explicitly crash, but is broken (missing file)
test_load_urdf_akb.py	- [ ]	Crashes, possibly due to missing file or path bug
test_load_urdf.py	- [ ]	Crashes on ur5 actions
test_object_data.py	- [x]	Passes in 2 seconds
test_ompl.py	- [ ]	Crashes because missing OMPL (but proper error not caught)
test_pick_and_place.py	- [x]	Passes in 19 seconds
test_ply_render.py	- [x]	Passes in 1 second
test_point_cloud_with_intrinsic_matrix.py	- [ ]	Crashes with segfault
test_point_cloud.py	- [ ]	Crashes with segfault
test_rigidbody_link.py	- [x]	Passes in 2 seconds
test_save_gripper.py	- [x]	Passes in 2 seconds
test_save_obj.py	- [x]	Passes in 1 seconds
test_scene.py	- [x]	Passes in 1 seconds
test_stretch3_move.py	- [x]	Passes in 24 seconds
test_tobor_move.py	- [x]	Passes in 24 seconds
test_urdf_parameter.py	- [x]	Passes in 1 second

After getting the tests nominally passing, I would also like to go through and clean them up:

• Remove uncommented code
• Add assertions and remove prints
• See if we can avoid the sys.path.append stuff at the top of some of the files
• Consider moving the location of the tests
• Clean up some of the verbose outputs (e.g., "Internal: deleting an allocation that is older than its permitted lifetime of 4 frames (age = 7)")