What is 2014 'UK English test scandal' over which Indian students are demanding action?

Students, including many Indians, have asked UK PM Rishi Sunak to act against the "unjust" revocation of their visas following an English test scandal in 2014. Investigation showed cheating had occurred at two centres for a compulsory test required for visas. The UK had launched a crackdown on the centres and terminated the visas of over 34,000 overseas students.