perf: replace redundant allocations with Linq

[TimothyMakkison]

Replace redundant repeat allocations with Linq. TUnit has a lot of code that replaces a 24 byte Linq allocation (sometimes Linq doesn't even allocate) with a O(n) temporary collection allocation.

There are way more of these for instance 0b1df1a, I don't think the AI understand Linq, as a rule of thumb, any comment starting with the word Optimize or mentions the word Linq is suspect

[thomhurst]

Thanks!