GL: that's fancy casts for you, you get CRASHES.

Sigh. I was really sure I commited this one back in
cc608dba42, but apparently not.

src/Magnum/GL/Test/DebugOutputGLTest.cpp

@@ -137,7 +137,7 @@ void DebugOutputGLTest::setCallbackDeprecated() {
         DebugOutput::setDefaultCallback();
     }};
 
-    Containers::String out;
+    std::ostringstream out;
     CORRADE_IGNORE_DEPRECATED_PUSH
     DebugOutput::setCallback([](DebugOutput::Source source, DebugOutput::Type type, UnsignedInt id, DebugOutput::Severity severity, const std::string& string, const void* userPtr) {
         Implementation::defaultDebugCallback(source, type, id, severity, string, static_cast<std::ostringstream*>(const_cast<void*>(userPtr)));
@@ -148,7 +148,7 @@ void DebugOutputGLTest::setCallbackDeprecated() {
         1337, DebugOutput::Severity::High, "Hello from OpenGL command stream!");
 
     MAGNUM_VERIFY_NO_GL_ERROR();
-    CORRADE_COMPARE(out,
+    CORRADE_COMPARE(out.str(),
         "Debug output: high severity application marker (1337): Hello from OpenGL command stream!\n");
 }
 #endif