If you have xdebug enabled and want to get the ui report of unit tests you can modify /fuel/core/phpunit.xml and add in the logging node like:
<logging>
<log type="coverage-html" target="path-to-report" charset="UTF-8" yui="true" highlight="false" lowUpperBound="35" highLowerBound="70"/>
</logging>
Then run unit tests as normal in fuelphp e.g. php oil test --group=Core.
After the tests are run you will see the line "Generating code coverage report, this may take a moment" to know you've been successful.