The vertical line between 8 and comment will are not completely connected top.
\begin{table}[H]
\centering
\begin{tabular}{|c|c|c|c|c|c|c|c|c|c|c|}
\hline
\multirow{2}{*}{No.} & \multirow{2}{*}{Date} & \multicolumn{8}{c}{Performed Test} & \multirow{2}{*}{Comment}\\
& & 1 & 2 & 3 & 4 & 5 & 6 & 7 & 8 & \\\specialrule{.2em}{.1em}{.1em}
\end{tabular}
\caption{Chronology of Performed Tests.}
\label{Tests}
\end{table}
