rostest: rostest depends on rostest-native (resolves #83)

The rostest package requires that the rostest executable can be
found by cmake's find during configure. Hence, rostest depends on
rostest-native.
To implement this, rostest and rostest-native are defined with
different dependencies.

This commit resolves issue #83.
Merge pull request #97 from bulwahn/rostest-depends-on-rostest-native-v2

rostest: rostest depends on rostest-native (resolves #83)