are there jobs for something similar except the vulnerabilities are to be found in the constitution, laws, etc.?
Yes, but they (1) take forms that are more subtle than straight-forward labels such as pentesting, and (2) usually are intended for taking advantage of detected loopholes rather than reporting them for a fix.
Fiscal engineering is a good example of vulnerability search. Short of incurring tax fraud, the task of financial engineering entails advanced knowledge of tax law so as to take advantage of the loopholes therein and thus minimize the amount that has to be paid to the tax authority.
Unlike computer pentesting, though, legislation very often requires or at least makes room for discretion on the part of the agent (whether it is a prosecutor, a judge, an analyst, and so forth) provided that the agent's decision is consistent with the legislative intent behind the statute(s) at issue. Accordingly, the matter of statutory loopholes is not as definite as the question of whether a computer system or program has bugs in it.