Math: actually test the conversion and not some random copy constructor.

src/Magnum/Math/Test/RectangularMatrixTest.cpp

@@ -211,7 +211,7 @@ void RectangularMatrixTest::convert() {
     constexpr Matrix2x3 b(Vector3(1.5f, 2.0f, -3.5f),
                           Vector3(2.0f, -3.1f,  0.4f));
 
-    constexpr Matrix2x3 c(b);
+    constexpr Matrix2x3 c{a};
     CORRADE_COMPARE(c, b);
 
     constexpr Mat2x3 d(b);

src/Magnum/Math/Test/VectorTest.cpp

@@ -234,7 +234,7 @@ void VectorTest::convert() {
     constexpr Vec3 a{1.5f, 2.0f, -3.5f};
     constexpr Vector3 b(1.5f, 2.0f, -3.5f);
 
-    constexpr Vector3 c(b);
+    constexpr Vector3 c{a};
     CORRADE_COMPARE(c, b);
 
     constexpr Vec3 d(b);