Sublinear Homomorphism Property Testing Counter Example
Thanks to the help in the comments I was able to unfixate and find an answer to the question. I prove $\delta\left(f\right)\geq\frac{1}{2}-\frac{1}{2\sqrt{n}}$.

Let $h:\mathbb{Z}_{n}\to\mathbb{Z}_{n}$ be an homomorphism.

• If $h\left(1\right)=f\left(1\right)$ then $h\left(1\right)=1$ and h is the identity function, since $h\left(0\right)=0$ and $$h\left(x+1\right)=h\left(x\right)+h\left(1\right)=h\left(x\right)+1$$ due to homomorphisms definition and properties. We get $\delta\left(f,h\right)=1-\frac{1}{\sqrt{n}}\geq\frac{1}{2}-\frac{1}{2\sqrt{n}}$ (since they disagree on $n-\sqrt{n}$ values). 

• Otherwise, $h\left(1\right)\neq f\left(1\right)$, that is $h\left(1\right)\neq1$. Assume by contradiction that exists $x\in\mathbb{Z}_{n}$ such that $h\left(x+1\right)=h\left(x\right)+1$, but due to homomorphisms definition we get $$h\left(x+1\right)=h\left(x\right)+h\left(1\right)=h\left(x\right)+1$$ which is a contradiction to $h\left(1\right)\neq1$. Since our function $f$ maintains that $$h\left(x+1\right)=h\left(x\right)+1$$ for $x\in\mathbb{Z}_{n}\backslash\left\{ q\sqrt{n}|0\leq q\leq\sqrt{n}-1\right\}$ , which means $n-\sqrt{n}$ “bad” pairs of $x$ and $x+1$. This means that in order to get any homomorphism, including $h$, we have to change at least one value for each pair, which means at least $\frac{\sqrt{n}-1}{2}$ values changed for each interval $\left[q\sqrt{n},\left(q+1\right)\sqrt{n}\right)\cap\mathbb{Z}_{n}$ (where $0\leq q\leq\sqrt{n}-1$), otherwise due to to the pigeonhole principle we have two consecutive values that are unchanged which is again a contradiction since then $h\left(x+1\right)=h\left(x\right)+1$. So in total we have at least $$\sqrt{n}\frac{\sqrt{n}-1}{2}=\frac{n-\sqrt{n}}{2}$$ values changed, which means $$\delta\left(f,h\right)\geq\frac{1}{2}-\frac{1}{2\sqrt{n}}$$