So I haven't found a way to mark permanently one rule as a start rule, however in the right-click context menu for the grammar, there are two options that relate to ANTLR, one of them is 'test rule xyz', that did the job:
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…