Only test the event backends available on the system.

Only test the event backends available on the system.

Fix how the CMake project adds the tests using the different backends. At
first we tried to do it exactly as it's done in test/test.sh.

However, test.sh uses a special program test-init to decide if a given
backend is available or not before running the actual tests. Doing it this way
will not be possible using CMake. Since then we would have to have the
test-init executable compiled at the time we run CMake, to know what tests
we should add. (And since CMake generates the make/project files that
compiles the executables, there's a catch 22).

Instead of deciding what tests to run this way, we simply use the result
of the CMake system introspection (that figures out what backends are
available) to decide what backend tests to add.