I presume that when you write that you want to test x/(1-ylag)=1, you really mean that you want to test b(x) / (1 - b(ylag)) = 1. That is, you want to test whether the parameter for x divided by 1 minus the parameter for ylag is equal to 1. Note that
b(x) / (1 - b(ylag)) = 1 => b(x) = 1 - b(ylag) => b(x) + b(ylag) = 1;
You should be able to test b(x) + b(ylag) = 1 with:
test x + ylag = 1;
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