Variable test: slight improvements to the addVariableDuplicates test.
Variable test: slight improvements to the addVariableDuplicates test.

Also updated some comments in the addEquivalenceReturnsFalseProperly test.

cellml/libcellml,cellml/libcellml,nickerso/libcellml,nickerso/libcellml,hsorby/libcellml,nickerso/libcellml,hsorby/libcellml,nickerso/libcellml,cellml/libcellml,hsorby/libcellml,hsorby/libcellml,cellml/libcellml