unit tests: require utf8 for quadblitter

tests/visual.cpp

@@ -247,6 +247,7 @@ TEST_CASE("Visual") {
 
   // close-in verification of each quadblitter output EGC 
   SUBCASE("QuadblitterEGCs") {
+    if(enforce_utf8()){
       // there are 16 configurations, each mapping four (2x2) pixels
       int DIMX = 32;
       int DIMY = 2;
@@ -316,6 +317,7 @@ TEST_CASE("Visual") {
       }
       delete[] rgba;
     }
+  }
 
   CHECK(!notcurses_stop(nc_));
 }