minor fixes to Array.protoype.find tests

Changes:

Array.prototype.find_callable-predicate:
 * should not try to exhaustively check all things that could be callable -- that belongs in a test for IsCallable()
 * should just test whether a known callable object succeeds w/o typerror

Array.prototype.find_this-is-object:
 * only boxes in non-strict mode

Array.prototype.find_this-is-undefined:
 * 'this' is undefined in strict mode, global object in sloppy mode

Ref #108 ; comments?
OK, I will split it into two tests.