rpv: Fixes and improvements

[leroyvn]

Description

This PR fixes and upgrades the RPV BSDF plugin. Changes are as follows:

• sample() is fixed: The missing pdf division has been added. A corresponding unit test was added, as well as a plugin evaluation helper in a new test_tools.plugin module.
• A few more fixes and improvements were made (constructor, string repr, tests).
• The RPVBSDF test suite was overhauled (code deduplication).
• A system test checking that degenerating the RPV BRDF into a Lambertian BRDF yields the expected result.

Checklist

• The code follows the relevant coding guidelines
• The code generates no new warnings
• The code is appropriately documented
• The code is tested to prove its function
• The feature branch is rebased on the current state of the main branch
• I updated the change log if relevant
• I give permission that the Eradiate project may redistribute my contributions under the terms of its license

[leroyvn]

@wint3ria please tell me if this impacts your workflow. The eval() method is unchanged, so I'd be surprised if this would make a difference for you.

[leroyvn]

@nollety I added tests derived from #235 and took the opportunity to deduplicate a lot of the RPV test code. Please take a look, comments are welcome.

[leroyvn]

@nollety I'll merge, we'll adjust the test code later if you wish.