Fixes the following vet reports:

common/step_create_floppy_test.go:79: possible formatting directive in Fatal call
common/step_create_floppy_test.go:89: possible formatting directive in Fatal call
common/step_create_floppy_test.go:180: possible formatting directive in Fatal call
common/step_create_floppy_test.go:190: possible formatting directive in Fatal call