Fetch Gtest if not found by CMake

[JBludau]

I just think this makes it more convenient for devs if working on a machine that does not have it installed

[dalg24]

The current CMake minimum required version is quite old (3.16):

Cabana/CMakeLists.txt

Line 15 in 8fb4b3e

cmake_minimum_required(VERSION 3.16)

If you bump it to something more recent (>=3.25) you could achieve the same thing w/o the if-else branch form a single FetchContent command.

[JBludau]

@streeve what do you think of bumping the cmake version? But it might create problems for our downstream users