Remove test dependency on bst-plugins-experimental

Description

Removes the test dependency on bst-plugins-experimental.

Changes proposed in this merge request:

• Remove integration tests for plugins that have moved repos
• Refactor tests/format/variables/{defaults,overrides} into a single project, with using locally defined versions of the moved plugins. This way we don't need to change the tests, nor do they need to be updated when the plugins are updated. The two separate projects had to be consolidated into one as python was not happy about the duplication of the files, and we don't allow local plugins from directories outside the project root.
• Remove test dep on bst-plugins-experimental

This merge request, when approved, will close: #1102 (closed)