Non-functional fixes to exu_alu testbench

[RRozak]

It fixes a typo and changes an if statement to an assert. The if that is replaced contains a list of operations, which forces us to modify it whenever we add a test of new operation. All these operations are listed in conditions of if statements that are a few lines above. If an operation doesn't have a corresponding if statement, the result variable is None, which is checked by an assert, which I added.

[github-actions[bot]]

Links to coverage and verification reports for this PR (#174) are available at https://chipsalliance.github.io/Cores-VeeR-EL2/

[tmichalak]

LGTM