test: add forall test for empty list and non-bool/non-function args

Add test case verifying that `forall` returns true for an empty list regardless of predicate, and add two new test files covering runtime errors when `forall` receives a non-boolean-returning function or a non-function argument. Also refactor the `forall` implementation to use logical negation (`!`) instead of explicit `!= true` comparison.

src/wren_core.c

@@ -60,7 +60,7 @@ static const char* libSource =
 "\n"
 "  forall(f) {\n"
 "    for (element in this) {\n"
-"      if (f.call(element) != true) return false\n"
+"      if (!f.call(element)) return false\n"
 "    }\n"
 "    return true\n"
 "  }\n"