Convert litehtml tests to use GoogleTest

GoogleTest provides a number of nice features (such as autodiscovery)
that make writing and running tests easier and less tedious.  This
patch converts the litehtml tests over to use GoogleTest.  Note that
the conversion is mostly mechanical -- no attempt has been made to
make the tests "idiomatic" GoogleTests.

Most of the CMake changes are based on code from the GoogleTest
documentation, specifically the "Quickstart: Building with CMake"
guide:

  https://google.github.io/googletest/quickstart-cmake.html