Do not raise an exception when bintest fail

- An exception do not raise when mrbtest fail.
- There are no useful informations in exception message and backtrace.

Fix typo in `AUTHORS` [ci skip]

Reduce `String` creation in `check_(cv|const)_name_sym`

Fix missing assertions in `mruby-string-ext` test

Update `AUTHORS` [ci skip]

Rename `MITL` to `LICENSE` in `.yardopts`; ref 67728c15 [ci skip]

Add bintest header

Add a missing file for #4314

Allow `enable_bintest` without `enable_test` in build config

Set `GEMNAME` on bintest

Refer also CXX and CC env vars as linker command in gcc and clang toolchain

ref #4292

Count skip tests

Extract similar code fragment to method in `src/class.c`

`GEMNAME` is undefined in bintest

ref: https://github.com/mruby/mruby/pull/4296#discussion_r261868710

Simplify `MRubyTestSkip` in `test/assert.rb`

Fix missing assertion in `test/t/class.rb`

To avoid memory leak from `time_update_datetime`.