Expands on unit test for rigid object asset class (#379)

# Description
We want to expand our test coverage, so this PR aims to increase
coverage of rigid object.

This PR ensures the following test cases are covered by unit tests for
rigid_object.py:
* external forces are applied correctly
* friction/mass/collider offsets randomization works correctly
* setting states work correctly
* reset works correctly

All test cases pass 👍 

Note: I wasn't able to find `collider offsets` in the code base but
happy to add a test for this too if someone can point me in the right
direction.

Fixes #363 

## Type of change

- New feature (non-breaking change which adds functionality)

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./orbit.sh --format`
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] I have run all the tests with `./orbit.sh --test` and they pass
- [ ] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [x] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

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---------
Signed-off-by: jsmith-bdai <142246516+jsmith-bdai@users.noreply.github.com>
Signed-off-by: James Smith <142246516+jsmith-bdai@users.noreply.github.com>
Co-authored-by: AutonomousHansen <50837800+AutonomousHansen@users.noreply.github.com>
Co-authored-by: Mayank Mittal <12863862+Mayankm96@users.noreply.github.com>