CSSTest.SelectorParse fails on GitHub Actions but not on Travis or on
any developer systems. Disable the test until we can determine what's
going on here.
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
