COMP: Allow CTest to cover innerbuild tests

With the superbuild architecture, the inner project
is not added as a subdirectory in the superbuild project,
but configured through ExternalProject_Add(). For this
reason, the CTestTestfile.cmake at the top level (superbuild
project) does not add the Innerbuild directory in which
there is another CTestTestfile.cmake referencing the tests
of the inner project.

This commit includes the file `imstkCTestAddInnerbuild.cmake.in`
which calls `subdirs()` on the Innerbuild directory. That file
is configured in the top-level build directory, and added as
the TEST_INCLUDE_FILE property to that directory. This will
include that file to the top-level CTestTestfile.cmake, which
will therefore be able to reach the CTestTestfile.cmake in the
Innerbuild directory when running ctest.

Use this commit to also only solve external dependencies
in the superbuild tree, no need to to it in the innerbuild tree.