Add OpenGL libraries to test target link libraries (#86).

[yanlinlin82]

Ensures that OpenGL libraries are linked when building test targets, which may be necessary for proper rendering and testing of graphics-related functionality.

[coalsont]

It is probably better to match wb_command:

workbench/src/CommandLine/CMakeLists.txt

Lines 81 to 93 in 4e0776f

	SET (MESA_OR_OPENGL_LIBRARIES "")
	IF (OSMESA_FOUND)
	SET(MESA_OR_OPENGL_LIBRARIES
	${OSMESA_OFFSCREEN_LIBRARY}
	${OSMESA_GL_LIBRARY}
	${OSMESA_GLU_LIBRARY})
	ELSE()
	IF (OPENGL_FOUND)
	SET(MESA_OR_OPENGL_LIBRARIES ${OPENGL_LIBRARIES})
	ELSE()
	MESSAGE(SEND_ERROR "Must have either OpenGL or Mesa3D for command line")
	ENDIF()
	ENDIF()