tests/frontend/workspace.py: Test that all elements build with workspace in play

- Tests that the target is still built even when a workspace is open
  on a runtime dependency of a build-only dependency.
- Tests that the target is still built even when a workspace is open
  on a runtime dependency of a runtime dependency of a build-only
  dependency

This adds the regression test provided by Matthew Yates for issue #919

This test differs from the one committed in master as:

- We have an orthogonal bug in 1.2.x where buildable elements show up
  in a waiting state instead of a buildable state
- Some new APIs were used in the test case added in master, adjusted
  to use only 1.2 APIs for the test case.
23 files changed