Add support for test-only dependencies in pyqt.mk

Allow ports to only depend on a component of PyQt for testing.

USES=pyqt:5
USE_PYQT=webengine_test

will add a TEST_DEPENDS on www/py-qt5-webengine

Approved by: portmgr (mat)
Differential Revision: https://reviews.freebsd.org/D23241