[experimental] test coverage using SimpleCov

I ported simplecov into ruby core for test coverage. 

### Usage

```shell
COVERAGE=1 make test-all TESTS=test/cgi
```

it generate test coverage to ```coverage``` direcotory.

### Limitation

 * test_coverage.rb and test_process.rb broke test suit with SimpleCov
 * some tests failed randomly.

### TODO

 * more reduce bundled simplecov(additional configuration, formatter, etc)
 * measure rubyspec coverage

/cc @colszowka