Check for FEATURETEST before running pytest

Check for FEATURETEST before running pytest

When compiling with meson, it may be easy to forget to compile system
test dependencies before running the tests. In that case, the test
results would be quite incosistent and unpredictable, with some tests
ending up with ERROR, some with FAILURE and others PASS, without a clear
indication that something is off before running the entire machinery.

Add a check to fail early on if the FEATURETEST binary isn't available,
indicating that system test dependencies were most likely not compiled.