Create random tests in TimeSheetArithmeticTest class

[gstuer]

Just for documentation purposes

If it says that we need those random tests we should make an issue for them and solve this problem :)

Originally posted by @gstuer in #79

[larsk21]

Good idea. Would it be helpful to wait until we merged #74?

[gstuer]

Yes you're completely right! This issue should be blocked until #74 is merged.