Include the following changes too:
- Extract part of logic with block to a method
- Use `var ||= ...` instead of `var = ... unless var`
- Unify using parentheses for `t_print`
- Change methods order
Can access to current gem's name through GEMNAME from test.
Can change assert library each mrbgems. used spec.test_preload on mrbgem.rake.
assert library should set $ok_test, $ko_test, $kill_test, $asserts.