GL: prevent a test crash on SwiftShader.

There's much more. Giving up for now.

src/Magnum/GL/Test/CubeMapTextureGLTest.cpp

@@ -1399,6 +1399,8 @@ void CubeMapTextureGLTest::generateMipmap() {
     CORRADE_COMPARE(texture.imageSize(1), Vector2i( 0));
     #endif
 
+    MAGNUM_VERIFY_NO_GL_ERROR();
+
     texture.generateMipmap();
 
     MAGNUM_VERIFY_NO_GL_ERROR();