Test cleanup.

src/Magnum/Test/ResourceManagerTest.cpp

@@ -47,11 +47,10 @@ struct ResourceManagerTest: TestSuite::Tester {
     void loader();
 };
 
-class Data {
-    public:
+struct Data {
     static std::size_t count;
 
-        Data() { ++count; }
+    explicit Data() { ++count; }
     ~Data() { --count; }
 };