Systematically test xvalidation of machines

[karlnapf]

Just like #3537, we want to make sure x-validation works for all Shogun machines.

For that, we need to write an automatically generated set of tests, similar to the nice patch #3679

Keep in mind we need

• single threaded
• multi-threaded

Expect memory errors.
In addition, as in the discussion of #3679 all the unit tests that are automatically generated for all Shogun classes should be separated out into a new binary, so that we can run these (expensive to compile and run) tests separately on the buildbot

[karlnapf]

#3742

[karlnapf]

Should be done in a similar way to https://github.com/shogun-toolbox/shogun/blob/develop/tests/unit/base/trained_model_serialization_unittest.cc.jinja2