test: add {if_not_exists = true} to box.schema.user.grant

Add {if_not_exists = true} to box.schema.user.grant to prevent
error when server restart.

test/box/gh-5645-several-iproto-threads.lua

@@ -13,7 +13,7 @@ box.cfg({
     wal_mode = 'none'
 })
 
-box.schema.user.grant('guest', 'read,write,execute,create,drop', 'universe')
+box.schema.user.grant('guest', 'read,write,execute,create,drop', 'universe', nil, {if_not_exists = true})
 function errinj_set(thread_id)
     if thread_id ~= nil then
         box.error.injection.set("ERRINJ_IPROTO_SINGLE_THREAD_STAT", thread_id)