Continuing moving plugins over to bst-plugins-experimental.

Continuing moving plugins to bst-plugins-experimental.

It was agreed on the mailing list to move all plugins to 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 repo.

This commit starts the process of moving things over by moving only the
cmake plugin to bst-plugins-experimental, and altering the tests to
reflect the new location.

When the missing variable was not defined, we would get an exception
as a string doesn't contain provenance information.

- Add a test to prevent regression

Note: This modifies the docker containers used for testing to supply the
pytest-timeout package.

This uses the build tool mode of cmake to have a single command that can call
either make or ninja.

I've also modified the tests to take the new commands into account