Check if GTest is available before downloading

[bkmgit]

Enable use of GTest that is already available.

[bkmgit]

This is similar to #1 though gmock needs to be linked in a similar manner to gtest_main on newer CMake versions.

[aiksiongkoh]

Does your PR supersede
#1

[chennes]

I think we should be using the QUIET option to the find_package command there, right?

[bkmgit]

Added QUIET option as done in #1, can rebase changes on top of #1 if that would be better.

[chennes]

@bkmgit if this is still something you're interested in pursuing, can you rebase? FreeCAD itself is going to drop all the FetchContent calls and simply make gtest a system requirement if you want to build the tests, so we could also make that change here, instead.

[bkmgit]

@chennes Done.

[chennes]

@aiksiongkoh do you want to test this to make sure it works on your development environment before we merge it?

[PaddleStroke]

@chennes I don't think that he'll feedback on that. I think Aik-Siong has a dev version of the solver that diverged significantly and he's not so proficient in handling development environment. So I'd venture a guess that he does not know how to test this. So if this change is needed you should probably go ahead and merge it.