Use dev version for examples when running tests

[hellemo]

Using a heuristic for whether the code is checked out and not installed is to check if the containing folder includes the full package name, ending with .jl.
Let's see if it works on github. I'm sure there is a better way to check if a package is installed or checked out? The idea is to develop the package if checked out locally or in CI, and use the registered version otherwise.

[JulStraus]

It seems to me, that with this approach, we may end up again at the old problem with write permissions when one used ] add.

[hellemo]

What was the problem with write permissions? That you were not allowed to add the Manifest file? Could we run it in a temporary environment, perhaps?

[JulStraus]

Rethinking about it, it should work. However, we need to:

1. Update the NEWS.md and
2. Increase the minor version.

If it is not working, we can reverse it in another minor increase. However, we need to register it to be certain that it is working.

[hellemo]

We concluded in offline discussion that we prefer to set a flag in ENV to toggle use of development version explicitly for tests.
Fixes #17

[JulStraus]

Looks good to me. We can use the same approach in all other packages in my opinion.