GL: make the Context::makeCurrent() test more robust.

So if it fails somewhere, the current context is reset back.

src/Magnum/GL/Test/ContextGLTest.cpp

@@ -242,9 +242,11 @@ void ContextGLTest::constructConfiguration() {
 
 void ContextGLTest::makeCurrent() {
     CORRADE_VERIFY(Context::hasCurrent());
+    Context* current = &Context::current();
 
-    Context& current = Context::current();
+    {
         Context::makeCurrent(nullptr);
+        Containers::ScopeGuard resetCurrent{current, Context::makeCurrent};
 
         CORRADE_VERIFY(!Context::hasCurrent());
 
@@ -257,11 +259,10 @@ void ContextGLTest::makeCurrent() {
         }
 
         CORRADE_VERIFY(!Context::hasCurrent());
-
-    Context::makeCurrent(&current);
+    }
 
     CORRADE_VERIFY(Context::hasCurrent());
-    CORRADE_COMPARE(&Context::current(), &current);
+    CORRADE_COMPARE(&Context::current(), current);
 }
 
 #ifndef CORRADE_TARGET_EMSCRIPTEN