Added photo_interrupter to the test

tests/photo_interrupters.scad

@@ -0,0 +1,28 @@
+//
+// NopSCADlib Copyright Chris Palmer 2018
+// nop.head@gmail.com
+// hydraraptor.blogspot.com
+//
+// This file is part of NopSCADlib.
+//
+// NopSCADlib is free software: you can redistribute it and/or modify it under the terms of the
+// GNU General Public License as published by the Free Software Foundation, either version 3 of
+// the License, or (at your option) any later version.
+//
+// NopSCADlib is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
+// without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+// See the GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License along with NopSCADlib.
+// If not, see <https://www.gnu.org/licenses/>.
+//
+include <../utils/core/core.scad>
+include <../vitamins/photo_interrupters.scad>
+
+use <../utils/layout.scad>
+
+module photo_interrupters()
+    layout([for(l = photo_interrupters) pi_base_length(l)], 5)
+        photo_interrupter(photo_interrupters[$i]);
+
+photo_interrupters();