This moves the tests section as is from CMakeLists.txt into
tests/tests.cmake. CMakeLists.txt now includes tests/tests.cmake
if the latter file exists.
Now it's possible to delete the whole "tests" directory and
building with CMake will still work normally, just without
the tests. This way the tests are readily available for those
who want them, and those who won't run the tests anyway have
a straightforward way to ensure that nothing from the "tests"
directory can affect the build process.
(cherry picked from commit aaff75c348)
Lasse Collin