Your test code shows single-register STORE/FETCH operations working, but the address is incorrect so the STORE/FETCH access the wrong scratch register. Since they both error identically, the test passed.
By single-register, I mean "direct" as opposed to "indirect". The indirect versions of STORE/FETCH work properly.
Direct versions (don't work): STORE s0, 5 FETCH s1, 7
Indirect versions (working): STORE s0, s2 FETCH s0, s2