By John Davison and Tom Perry BAGHDAD (Reuters) -A power struggle in Iraq between the Shi’ite cleric Moqtada al-Sadr and rivals aligned with Iran is testing Tehran’s ability to stave off a conflict that could damage its interests and further destabilise the oil-rich country. With Sadr supporters camped out in parliament and his opponents protesting in the streets, the tussle over a new government has put fresh strain on a political system that has been buffeted by crises since U.S.-led forces toppled dictator Saddam Hussein two decades ago. It adds another destabilising factor to a belt of fra…