The function of ${\mathrm{ZnCl}}_2$ in Lucas test for alcohols is 

Lucas test : In the presence of anhydrous ${\mathrm{ZnCl}}_2$ alcohol reacts with cone. $\mathrm{HCl}$ to produce alkyl chloride. 

$\mathrm{ROH}+\mathrm{HCl}\stackrel{{\mathrm{ZnCl}}_2}{⟶}\mathrm{RCl}+{\mathrm{H}}_2\mathrm{O}$

Mechanism of Lucas test

$\begin{array}{l}2\mathrm{HCl}+{\mathrm{ZnCl}}_2⟶{\mathrm{H}}_2{\mathrm{ZnCl}}_4\\ \mathrm{ROH}+{\mathrm{H}}_2{\mathrm{ZnCl}}_4⟶\mathrm{RCl}+{\mathrm{ZnCl}}_2+{\mathrm{H}}_2\mathrm{O}+\mathrm{HCl}\\ \mathrm{\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_}\\ \mathrm{ROH}+\mathrm{HCI}\stackrel{{\mathrm{ZnCl}}_2}{\to }\mathrm{RCI}+{\mathrm{H}}_2\mathrm{O}.\\ \mathrm{\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_}\end{array}$