The Lassaigne's extract is boiled with dil. ${\mathrm{HNO}}_3$ before testing for halogens because

Silver halides are soluble in ${\mathrm{HNO}}_3$

${\mathrm{Na}}_2\mathrm{S}$ and NaCN are decomposed by ${\mathrm{HNO}}_3$

${\mathrm{Ag}}_2\mathrm{S}$ is soluble in ${\mathrm{HNO}}_3$

AgCN is soluble is ${\mathrm{HNO}}_3$