Try adding % \\ after the & in each of the lines that end with &: it seems to defeat the indentation engine without causing any problems.:
... \multicolumn{1}{c}{Testing...} & % \\ {\begin{minipage}{2cm}\includegraphics...\end{minipage}} & % \\ {\begin{minipage}{2cm}\includegraphics...\end{minipage}} & % \\ {\begin{minipage}{2cm}\includegraphics...\end{minipage}} \\ ... Very lightly tested.