Test: consistently use a struct for Tester subclasses.

I thought I cleaned this up ages ago, heh.

src/Magnum/Trade/Test/FlatMaterialDataTest.cpp

@@ -33,8 +33,7 @@
 
 namespace Magnum { namespace Trade { namespace Test { namespace {
 
-class FlatMaterialDataTest: public TestSuite::Tester {
-    public:
+struct FlatMaterialDataTest: TestSuite::Tester {
     explicit FlatMaterialDataTest();
 
     void baseColor();

src/Magnum/Trade/Test/MaterialDataTest.cpp

@@ -43,8 +43,7 @@
 
 namespace Magnum { namespace Trade { namespace Test { namespace {
 
-class MaterialDataTest: public TestSuite::Tester {
-    public:
+struct MaterialDataTest: TestSuite::Tester {
     explicit MaterialDataTest();
 
     void textureSwizzleComponentCount();

src/Magnum/Trade/Test/ObjectData2DTest.cpp

@@ -34,8 +34,7 @@
 
 namespace Magnum { namespace Trade { namespace Test { namespace {
 
-class ObjectData2DTest: public TestSuite::Tester {
-    public:
+struct ObjectData2DTest: TestSuite::Tester {
     explicit ObjectData2DTest();
 
     void constructEmpty();

src/Magnum/Trade/Test/ObjectData3DTest.cpp

@@ -34,8 +34,7 @@
 
 namespace Magnum { namespace Trade { namespace Test { namespace {
 
-class ObjectData3DTest: public TestSuite::Tester {
-    public:
+struct ObjectData3DTest: TestSuite::Tester {
     explicit ObjectData3DTest();
 
     void constructEmpty();

src/Magnum/Trade/Test/PbrClearCoatMaterialDataTest.cpp

@@ -32,8 +32,7 @@
 
 namespace Magnum { namespace Trade { namespace Test { namespace {
 
-class PbrClearCoatMaterialDataTest: public TestSuite::Tester {
-    public:
+struct PbrClearCoatMaterialDataTest: TestSuite::Tester {
     explicit PbrClearCoatMaterialDataTest();
 
     void basics();

src/Magnum/Trade/Test/PbrMetallicRoughnessMaterialDataTest.cpp

@@ -34,8 +34,7 @@
 
 namespace Magnum { namespace Trade { namespace Test { namespace {
 
-class PbrMetallicRoughnessMaterialDataTest: public TestSuite::Tester {
-    public:
+struct PbrMetallicRoughnessMaterialDataTest: TestSuite::Tester {
     explicit PbrMetallicRoughnessMaterialDataTest();
 
     void basics();

src/Magnum/Trade/Test/PbrSpecularGlossinessMaterialDataTest.cpp

@@ -34,8 +34,7 @@
 
 namespace Magnum { namespace Trade { namespace Test { namespace {
 
-class PbrSpecularGlossinessMaterialDataTest: public TestSuite::Tester {
-    public:
+struct PbrSpecularGlossinessMaterialDataTest: TestSuite::Tester {
     explicit PbrSpecularGlossinessMaterialDataTest();
 
     void basics();

src/Magnum/Trade/Test/PhongMaterialDataTest.cpp

@@ -34,8 +34,7 @@
 
 namespace Magnum { namespace Trade { namespace Test { namespace {
 
-class PhongMaterialDataTest: public TestSuite::Tester {
-    public:
+struct PhongMaterialDataTest: TestSuite::Tester {
     explicit PhongMaterialDataTest();
 
     #ifdef MAGNUM_BUILD_DEPRECATED

src/Magnum/Trade/Test/TextureDataTest.cpp

@@ -31,8 +31,7 @@
 
 namespace Magnum { namespace Trade { namespace Test { namespace {
 
-class TextureDataTest: public TestSuite::Tester {
-    public:
+struct TextureDataTest: TestSuite::Tester {
     explicit TextureDataTest();
 
     void construct();