Revert "Units test: proper way of testing coverage for UnitsItem::isValid()."
Revert "Units test: proper way of testing coverage for UnitsItem::isValid()."

This reverts commit 70796228ad21f44d7962e03b0f2b78a455c323d4.

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