Remove some clippy::all uses from UI tests (#14600)

It's either unneeded (`warn`/`deny`) or can be replaced by individual
lints (`allow`). Also removes some redundant `allow`s covered by the
default `-Aunused` that I saw along the way

changelog:  none

Author: Jarcho

tests/ui/author/if.rs

@@ -1,6 +1,6 @@
 //@ check-pass
 
-#[allow(clippy::all)]
+#![allow(clippy::all)]
 
 fn main() {
     #[clippy::author]

tests/ui/box_collection.rs

@@ -1,4 +1,3 @@
-#![warn(clippy::all)]
 #![allow(
     clippy::boxed_local,
     clippy::needless_pass_by_value,