In ODIs where DRS is used, each side is allowed at most one unsuccessful player review in an inning. What is the corresponding rule for test cricket?
In ODIs where DRS is used, each side is allowed at most two unsuccessful player reviews in an inning.
This is incorrect. Quoting from the current ODI playing conditions, Appendix 6, Section 3.5(a):
Each team is allowed to make one unsuccessful Player Review requests per innings.
(my emphasis). Exactly the same section in the current Test Match playing conditions answers your question:
each team is allowed to make a maximum of two unsuccessful player review requests in the first 80 overs of the innings, and a maximum of two unsuccessful player review requests after 80 overs for the remainder of the innings.