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.

CCBUG: 445259