Re-enable testing of RollbackableColor plugin

Problem/Motivation

To do a D11 compatible release, I had to remove drupal/color from test dependencies since it does not have any D11-compatible release. This also means that I have to skip testing this destination plugin on every Drupal core version.

This task is about to re-add this dependency to composer.json and removing some lines from RollbackableColorTest.

Remaining tasks

1. Create an MR.
2. Wait for a Drupal 11 compatible Color release.

API changes

No changes.