Test dependencies are not "real" code dependencies, they just
declare that for testing the plugin the other plugin has to
be loaded, even though it is _not_ a code dependency.
Add PLUGIN_TEST_DEPENDS to add_qtc_plugin. We cannot assume
that the target exists at that point, so adapt the usual pattern.
Fixes: QTCREATORBUG-25024
Change-Id: I4165ff8df762309e0be0bfe9e8bedef796a3bf17
Reviewed-by: Christian Stenger <christian.stenger@qt.io>
Reviewed-by: Cristian Adam <cristian.adam@qt.io>