Usage
The make task `update-coverage' retrieves simplecov for coverage report.
COVERAGE=1 make test-all TESTS=test/cgi
it generates test coverage to coverage directory.
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