Makefile: make sure test helpers are rebuilt when headers change
Makefile: make sure test helpers are rebuilt when headers change

It is not worth the bother to maintain an up-to-date list of
which headers each test helper uses, so depend on $(LIB_H) to
catch them all.

Signed-off-by: Jonathan Nieder <b57189c5f2fd5b3daf3350a77fde90def1080fa5@gmail.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git