vecmath: Update to zig 0.16.0, harden tests and add test build step

packages/vecmath/src/vectors/Vector2x8.zig

@@ -231,4 +231,8 @@ pub const Vector2x8 = struct {
             .y = self.x * vm.ps(m.iy) + self.y * vm.ps(m.jy),
         };
     }
+
+    test "refAllDecls" {
+        std.testing.refAllDecls(@This());
+    }
 };