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.

invalid base shouldn't silently fail

remove unused `mrb_str_body`

Revert 44a58092 and check range of
`base` with `mrb_assert`.
It actually can only be either 2, 8, 10 or 16 (and never 0), unless
mruby sources are modified. But I don't see a reason to impose further
limits.

Use `mrb_print_error` in test error handling.

Remove unnecessary out of memory check in `ary_expand_capa`.

Add new API `mrb_ary_resize`(mruby implementation of `rb_ary_resize`).

Since `mrb_realloc` raises when failed to allocate memory.

Reduce allocation in shared RArray clearing.

Fix type checking in `mrb_flo_to_fixnum`.

Always return non-NULL in `mrb_hash_tbl`.

Use RSTR_NOFREE_P in state.c

Use *_EMBED_* macro in state.c

Add method `toolchains` and `primary_toolchain` to `MRuby::Build`.