Add news item for v3.6.7

Author: nanxstats

NEWS.md

@@ -5,6 +5,10 @@
 - All plots in vignettes are now generated by the native SVG device
   for shaper apperance and fewer package dependencies (@nanxstats, #188).
 
+## Testing
+
+- Add independent unit tests for `toInteger()` (@DMuriuki, #186).
+
 # gsDesign 3.6.6 (February 2025)
 
 ## New features