Plugin marco changes

Add PDAL_ADD_TEST cmake macro, for use by plugins to create new test executables. Also, tweak PDAL_ADD_PLUGIN macros to *not* include PCL_LIBRARIES and *do* include PDAL_LINKAGE.

These were plucked from https://github.com/PDAL/PDAL/tree/issues/554-port-all-plugins for use by #567, with the idea of rebasing **issues/554-port-all-plugins** onto these changes.