Add copyright header in tests source files

The tests are starting to become important as the project is starting to implement a quite good amount of tests.

I do not know if this is a good practice to add the copyright header statement in the test source files.

But, I assume this is still some code which validate, lock the runtime code, which is part of the project with copyright on it.

This is surely a prerequisite for Debian packaging, since it was necessary to add this headers in the runtime code. At that time, there were almost no tests. Today, there is.