ENH: Create a functional install target

Update Artichoke to commontk/Artichoke@643f0af42075. Use new
ExternalProject_Install_CMake to install external projects when doing a
superbuild build of slicer, so that there is a functional install target
for the same.

This change is made synchronously with a change to CTK to stop
installing QtTesting to the build tree and instead install it as part of
the install target for CTK; thus, SlicerBlockInstallQtTesting.cmake is
(and indeed, must be, as there is no longer an install path being
exported) removed.

At least on Fedora, this is sufficient to produce a Slicer that can be
successfully run after building the 'install' target.