[428434] Provide higher-level test APIs

This adds a new feature, org.eclipse.sirius.tests, with two plug-ins:
* org.eclipse.sirius.tests.support: APIs and helpers for JUnit-based
  tests.
* org.eclipse.sirius.tests.swtbot.support: APIs and helpers for
  SWTBot-based tests.

Change-Id: I4b9f74146d405b169b1e4b413874530bd192b3db
Signed-off-by: Pierre-Charles David <pierre-charles.david@obeo.fr>
[428434] Fix target platform definitions

A misconfiguration in the .targetplatform files in the previous
commit (missing "with requirements" option in the SWTbot section) caused
the resulting .target files to switch from planner mode to slicer mode.
This broke the build on Juno (but strangely not for Kepler and Luna).

Change-Id: I18b7dd2afab809df34f3ea53921adc8526d9bbf8