autotests: Make decoration mode change tests more robust
Currently, kwin expects that the xdg-decoration is installed before the initial commit. However, decoration tests do that after the initial commit, which makes testMaximizeAndChangeDecorationModeAfterInitialCommit() silently pass.
On a second look, it seems that the xdg-decoration spec is okay with the xdg-decoration being created after the first commit (as long as it's done before the surface is mapped). This needs to be fixed separately.