ENH: Create a functional install target

Update Artichoke to commontk/Artichoke@b0cf6fc85d5a. 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 the merge of commontk/CTK#466
which no longer installs QtTesting to the build tree, and instead
installs 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.