The test codes are written using SimpleTest framework, and I am looking for a tool or an open source solution to perform Code Coverage.
You can use phpunit
Read about it, mostly used.
http://phpunit.de -- page is in english
Git: https://github.com/sebastianbergmann/phpunit
CodeCoverage: http://phpunit.de/manual/current/en/code-coverage-analysis.html
Use SimpleTest's Coverage extension:
https://github.com/simpletest/simpletest/tree/master/extensions/coverage