Move word boxing structs to "mruby/boxing_word.h".

Print backtrace of crashed test in verbose mode.

Check mrbgem.rake instead of directory to ignore empty directories in full-core gembox.

Use `mrb_bool` as return type of `mrb_const_defined_at`.

Inherit `$mrbtest_verbose` flag in core tests.

use C style comments instead of C++ style comments

Fix rake failure since `mrbtest_objs` isn't flattened.

Append generated test object to `@test_objs` of mrbgem spec.
Add method `custom_test_init?` to check whether mrbgem has custom test init function.

Solves #2355.

In test drivers:
* Uses `mrb_t_pass_result` to check and pass test result to main `mrb_state`.
* Adds `mrb_init_test_driver` to init test `mrb_state`.

Previous version ignored some errors, and didn't free memory and close files.
Now no memory will be dynamically allocated to simplify error handling.

This commit also fixes a wrong check:

  files[i] = fopen(argv[i], "wb");
  if (!ireps[i]) {

Improve error messages a bit and add missing newline to one.

Support minimum dependency test.

Solves #2355.

In test drivers:
* Uses `mrb_t_pass_result` to check and pass test result to main `mrb_state`.
* Adds `mrb_init_test_driver` to init test `mrb_state`.