How to write side-note reviews on assignments without sounding testy?
I can see you've given some thought to balancing the negative with some positive.  That's good.  Note, you don't need to remark on every single thing they did right.

If you have a rubric, a list of elements you look for in each lab report, you may wish to put a check mark in the margin wherever you see that a required element is present.

Sometimes a student doesn't manage to get much of anything right.  In that case, it can be helpful to notice something positive about what they were trying to do, e.g. "Glad to see you caught this contradiction."
Try expressing things as questions, rather than statements.  Examples:


  Can you also check such-and-so?  How does this fit in with etc.?


One of my professors would write in my proofs, instead of "Does not follow" or "Not relevant,"


  How is this germane?

As a student, with certain graders, I was sometimes unsure whether a particular comment was positive or negative or just food for thought.  So try to avoid those misunderstandings, e.g. "Food for thought: etc." or "Enrichment idea: etc." or "Conclusion is correct; be careful to make clear the connection between…"
Be tactful — meaning, your belief that they are trying hard, and can get better with your guidance, should come through.  Example: "Watch out, need more digits here to avoid rounding error later on." 
I suggest you arrange with the professor to visit the class briefly at or near the beginning of the semester, and maybe at the midpoint again, to introduce yourself and show a friendly face.  Convey to them that your job is to give them lots of specific feedback to help them improve their work.  This can help humanize things.