Subjunctive. 'That,' ver. 20, refers to 'hereby' in ver. 19. It is showing what tests 'in truth.' The repetition of 'that' is nothing uncommon. It is found in Eph. 2.11,12. What 'hereby' refers to very commonly follows in John, as in vers. 16,24, and ch. 4.17, and elsewhere, frequently indeed with 'that.' 'God being greater' is evidently a testing, searching thing.