Use CMake to download GoogleTest as part of build

[mkilivan]

This change is to use CMake to download GoogleTest as part of the
build's configuration step.

[mkilivan]

Please note that KjellKod/g3log#355 should be merged before this pull request. This is why CI build failed.

GoogleTest is now available on the top level. So, it will be downloaded and used for once for all sub folders.

ADD_LOGROTATE_UNIT_TEST CMake option has been renamed to BUILD_TEST. And moved to top level.

[KjellKod]

This is a sweet change @mkilivan
KjellKod/g3log#355 is now merged. When you have time, please update this PR accordingly.

[KjellKod]

For some reason g3sinks notification did not reach me. Github's notification to me is still wonky :(

Not sure what's going on here with this PR but if I run this :

cd build; cmake -BUILD_TEST=ON ..

It creates a directory "UILD_TEST" and puts all build artifacts in there.

If I simply run

```cd build; cmake ..

Then all artifacts are, as expected in the build directory

[KjellKod]

Ooops. Never mind I built it poorly I forgot the D in cmake -DBUILD_TEST=ON ..

[KjellKod]

This works great! thanks for the improvement