Add actuator tests for Implicit, IdealPD, and DCMotor

[jtigue-bdai]

Description

This PR adds tests for actuator initialization, configuration, and computation for Implicit, IdealPD, and DC motor actuators

NOTE: This PR is dependant on the pytest migration in: #2034

Fixes #2139

Type of change

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

Screenshots

Please attach before and after screenshots of the change if applicable.

Checklist

• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have updated the changelog and the corresponding version in the extension's config/extension.toml file
• I have added my name to the CONTRIBUTORS.md or my name already exists there