WIP: Adds test for string comparison (==/equals)

[maxkratz]

Stalled by Echtzeitsysteme/gips#22.

[maxkratz]

Currently blocked by a compilation error in generated code of a mapping constraint:

	protected double builder_1(final N2nMapping context) {
		return -1.0 * context.getH().getName();
	}
	protected ILPTerm builder_0(final N2nMapping context) {
		return new ILPTerm(context, (double)context.getG().getName());
	}

See Echtzeitsysteme/gips#31.