testsuite: clear_current() when done

That way we ensure that the GL context(s) get disposed, which they
previously weren't due to them still being the current context.

This also implicitly adds testing of gLContext destruction, which
previously wasn't ever done by any test.

testsuite/gdk/memorytexture.c

@@ -649,6 +649,7 @@ main (int argc, char *argv[])
       gsk_renderer_unrealize (gl_renderer);
       g_clear_object (&gl_renderer);
     }
+  gdk_gl_context_clear_current ();
 
   return result;
 }

testsuite/gdk/texture-threads.c

@@ -108,6 +108,7 @@ texture_threads (void)
   g_clear_pointer (&loop, g_main_loop_unref);
   g_clear_object (&gl_renderer);
   g_main_context_release (NULL);
+  gdk_gl_context_clear_current ();
 }
 
 int