Add test for fill constructor

vector_test.cpp

@@ -33,6 +33,10 @@ TEST(Vector, DefaultConstructorIsAllZero) {
   EXPECT_THAT(vec3i(), (vec3i{{0,0,0}}));
 }
 
+TEST(Vector, FillConstructorInsertsAllElements) {
+  EXPECT_THAT(vec3i(1, math::vector::fill), vec3i({1, 1, 1}));
+}
+
 TEST(Vector, ExtensionConstructorAppendsZeros) {
   EXPECT_THAT(vec4i(iota3i()), (vec4i{{1,2,3,0}}));
 }