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?
I can't support this pattern without lots of convincing. It seems like it is just a hack to make the test always pass. Consider an implementation that doesn't support strict or sloppy mode - this test will always pass regardless. I would rather see a separate strict mode test.