Set strings as const strings where possible in tests.
Set strings as const strings where possible in tests.

cellml/libcellml,hsorby/libcellml,nickerso/libcellml,cellml/libcellml,hsorby/libcellml,nickerso/libcellml,MichaelClerx/libcellml,hsorby/libcellml,libcellml/buildbot-testing,hsorby/libcellml,nickerso/libcellml,cellml/libcellml,MichaelClerx/libcellml,libcellml/buildbot-testing,nickerso/libcellml,cellml/libcellml,MichaelClerx/libcellml