There is a question of semantics. The genitive 's' has various nuances controlled by context.
The genitive noun is a determiner that implies an association with its object noun. There are default associations.
A. Today's test was difficult.
= (i) The test that was set today was difficult.
= (ii) The test that was taken today was difficult.
There is no ambiguity here. Both express the difficulty of the test.
Compare
The headmaster's test was difficult =
(i) The test taken by the headmaster was difficult.
(ii) The test set by the headmaster was difficult.
Now replace "headmaster" with geography...
B. Geography's test was difficult.
= (i) * The test that was set by geography was difficult.
(ii) * The test that was taken by geography was difficult.
There have been attempts in earlier comments to understand "geography's" as "The subject associated with geography", but this use is not idiomatic and therefore the default association fails.
Idiomatically: "The geography test was difficult."