how do I test code agenst `catkin install`-ed directory

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All tests are only run against the devel space. So you can not run the unit tests of a specific package against the installed version of the same package.

What you can do is run the tests of a package foo against the installed version of all dependencies. Therefore you first build and install all dependencies using one workspace and then use a second workspace containing foo and run its tests. Usually those kind of tests are not unit tests but integration tests - you might consider to bundle integration tests into separate packages (which you could ignore when releasing a repo) for this purpose.