Use `mrb_bool` as return type of `mrb_const_defined_at`.
Set `$mrbtest_verbose` flag in `mrb_init_test_driver`.
Merge pull request #2464 from take-cheeze/inherit_verbose_flag

Inherit `$mrbtest_verbose` flag in core tests.
Merge pull request #2471 from take-cheeze/const_defined_at_ret

Use `mrb_bool` as return type of `mrb_const_defined_at`.
backtrace supression condition was wrong; fix #2472
fiber stack allocation size should be bigger than irep->nregs