Make install tests an standalone cmake application

[vicentebolea]

We want to change the testing/install/CMakeLists.txt so that it is its own standalone cmake project as opposed to being part of a cmake project.

          It will work at the level of testing/install at least, since these install tests run standalone via ctest. 

I tried it, adding the subdirs to one of these if blocks patched into testing/install/CMakeLists.txt. It works.
(need to set it up as if()... else() or else the existing custom tests fail).

Originally posted by @drew-parsons in #3795 (comment)