GL: harden multi-Context test.

src/Magnum/GL/Test/ContextGLTest.cpp

@@ -28,6 +28,7 @@
 #include "Magnum/GL/Context.h"
 #include "Magnum/GL/Extensions.h"
 #include "Magnum/GL/OpenGLTester.h"
+#include "Magnum/Platform/GLContext.h"
 
 #ifndef CORRADE_TARGET_EMSCRIPTEN
 #include <thread>
@@ -78,6 +79,15 @@ void ContextGLTest::makeCurrent() {
 
     CORRADE_VERIFY(!Context::hasCurrent());
 
+    {
+        const char* argv[]{"", "--magnum-log", "off"};
+        Platform::GLContext ctx{Int(Containers::arraySize(argv)), argv};
+
+        CORRADE_VERIFY(Context::hasCurrent());
+    }
+
+    CORRADE_VERIFY(!Context::hasCurrent());
+
     Context::makeCurrent(&current);
 
     CORRADE_VERIFY(Context::hasCurrent());