The Lassaigne’s extract is boiled with $$dil.\,HN{O_3}$$   before testing for halogens because

Solution :
$$N{a_2}S$$  and $$NaCN,$$  formed during fusion with metallic sodium, must be removed before adding $$AgN{O_3},$$  otherwise black $$ppt.$$  due to $$N{a_2}S$$  or white precipitate due to $$AgCN$$  will be formed and thus white precipitate of $$AgCl$$  will not be identified easily.
\[\begin{align} & N{{a}_{2}}S+2AgN{{O}_{3}}\to 2NaN{{O}_{3}}+\underset{\text{Black}}{\mathop{A{{g}_{2}}S\downarrow }}\, \\ & NaCN+AgN{{O}_{3}}\to NaN{{O}_{3}}+\underset{\text{White}}{\mathop{AgCN\downarrow }}\, \\ & NaCl+AgN{{O}_{3}}\to NaN{{O}_{3}}+\underset{\text{White}}{\mathop{AgCl\downarrow }}\, \\ & N{{a}_{2}}S+2HN{{O}_{3}}\xrightarrow{\text{boil}}2NaN{{O}_{3}}+{{H}_{2}}S\uparrow \\ & NaCN+HN{{O}_{3}}\xrightarrow{\text{boil}}NaN{{O}_{3}}+HCN\uparrow \\ \end{align}\]