Add iidm test directory in include path

Signed-off-by: Sébastien LAIGRE slaigre@silicom.fr

Please check if the PR fulfills these requirements (please use '[x]' to check the checkboxes, or submit the PR and then click the checkboxes)

[ ] The commit message follows our guidelines
[ ] Tests for the changes have been added (for bug fixes / features)
[ ] Docs have been added / updated (for bug fixes / features)

Does this PR already have an issue describing the problem ? If so, link to this issue using '#XXX' and skip the rest No

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...) Add tests/iidm in include path, because it contains NetworkFactory.hpp, which contains useful network for tests, but the file could not be included simply

What is the current behavior? (You can also link to an open issue here) NetworkFactory.hpp can be included by writing

#include "../NetworkFactory.hpp"

What is the new behavior (if this is a feature change)?

NetworkFactory.hpp can be included by writing

#include "NetworkFactory.hpp"

Does this PR introduce a breaking change or deprecate an API? If yes, check the following:

[ ] The Breaking Change or Deprecated label has been added
[ ] The migration guide has been updated in the github wiki (What changes might users need to make in their application due to this PR?)

Other information:

(if any of the questions/checkboxes don't apply, please delete them entirely)

powsybl / powsybl-iidm4cpp

Add iidm test directory in include path #429