By Yoruk Bahceli, Dhara Ranasinghe and Stefano Rebaudo LONDON (Reuters) – As the European Central Bank rushes to exit stimulus and raise interest rates to tame inflation, bond markets are testing its ability and willingness to act against the strains that are starting to hit weaker countries in the bloc. The premia investors demand to hold bonds from Italy, Spain and Portugal relative to safer German debt — spreads in market parlance — have risen to the highest since 2020. With ECB key rates seen rising by 75 basis points within the next three months, Italian and Spanish 10-year borrowing co…