CMake file prevents cross compilation

[ALPayne18]

The CMakeLists.txt file always enables tests when building the project. This requires dependency on gtest and the the build and run of the tests when building with Conan.

The problem is when you need to cross compile the project (for example for armv8), the tests can be compiled successfully but cannot be run on the build environment (i.e x86_64) since the tests are compiled for the host (i.e. armv8).

An option should be added to disable testing. Preferably this would disable both building and running the test code when cross compiling.