Instead of redundantly documenting "directory" configuration option in
each source plugin, refer to the built-in functionality documentation of
the Source class.

  * source.py: Add anchor for built-in functionality.
  * sources/plugins/*.py: Remove directory config option, and add reference to
    Source's built-in functionality.

Fixes #713.

For each element plugin derived from BuildElement, add reference to the
built-in functionality documentation of BuildElement.

  * buildelement.py: Add anchor for built-in functionality.
  * plugins/elements/*.py: Add refence to BuildElement's built-in
    functionality for each build element.

Related to #713.

plugins: Don't redundantly document common configurations

Closes #713

See merge request !896

This moves tests-unix to use the standard linux-tests template
by splitting differently {before,after,}script, allowing for less
redundancy