Move cmake plugin to bst-plugins-experimental

It was agreed on the mailing list 0 to move all plugins into a single repository, before moving them into domain specific repositories. As a result it seems reasonable to move everything to the bst-plugins-experimental repo as this stepping stone, rather than creating a whole new repository.

This commit starts the process of moving things over by moving only the cmake plugin along with its tests.

Using the testutils from BuildStream was causing a stack trace, so I switched to use the ones we have in this project. However, we should remove these altogether and use upstream.

This is obviously blocked on getting the cmake plugin moved into bst-plugins-experimental.