Improve buildbot dashboard submission.

This adds a new CMake variable (not exposed via any options — it should be set in the initial CMakeCache.txt) named SimMedTK_SUPERBUILD_SUBPROJECT_DASHBOARD. When set, the superbuild will not build the "ALL" target of the SimMedTK subproject but insteadl build the "Experimental" target, which runs tests and submits results to CDash.

This way, buildbot superbuilds can also be configured to run SimMedTK tests (plus any superbuild tests) and the results will be submitted as two separate CDash rows.

This commit also enables testing on the superbuild and adds a single dummy test.