ctest must not run against installed library

from Joachim, see here

This is a long-standing issue that resurfaces over and again in different projects. You do make, ctest, 
make install. Then you modify the code, do make, ctest, make install again - and your code is broken. 
Why? Because the second time you run ctest, it executes using the *installed* library, not the latest,
modified version from your build/lib directory, and so you overlooked a fatal bug your modfication 
introduced in the library.

As a developer, I can generally avoid this issue by having the relative path `lib` in the beginning of my
`LD_LIBRARY_PATH`. But as library maintainers, couldn't we also do something to prevent this kind of error?