Adding test ensuring we can load different plugins with the same name

This is important, since a Pipeline can recurse into other Pipelines,
it's important that the user provided plugins loaded for each respective
pipeline are clearly separated. Two projects may declare plugin 'foo' and
do so differently.

This test simply loads two clearly different plugins both named 'foo'
but does so in different plugin contexts, and tests that these plugins
are indeed different implementations for each context.