test case illustrating set! problem

(derived from drdr failure for
 scribblings/guide/contracts-examples/1-test.rkt)
improve the first-order checking for the current class/c implementation

(not strictly necessary, but the new, still pending class/c first-order
checking checks the arities of the methods and those additional
tests got put into the test suite, so the easiest thing for now
is just to make the current class/c implementation do that check too)