Use target path for executable

By using this form of the add_test() statement, CMake will automatically
fill in the path of the generated executable instead of letting this be
coded coincidentally the same as the target here.

Note that this does *not* create a dependency between the test and the
target -- it is simply assumed to already exist.
Build executables before running tests

Make sure that the executables are in place before running the tests,
so that it suffices to just run `make check` to build and test.

For the "tests" target to be created, we must call this auxiliary
module after OpmLibMain has setup its satellites.
Merge pull request #73 from rolk/073_check

`make check` behaves different from `make ; make check`