clippy-Deny useless vecs in test code

There's no reason to allow these, and now that they've been
removed, we should simply deny them in tests like we do in normal
code.

Author: TheBlueMatt

ci/check-lint.sh

@@ -110,4 +110,4 @@ CLIPPY() {
 
 CLIPPY
 # We allow some additional warnings in tests which we should fix, but which aren't currently a priority
-CLIPPY --tests "-A clippy::bool_assert_comparison -A clippy::assertions_on_constants -A clippy::needless-late-init -A clippy::field_reassign_with_default -A clippy::unnecessary_literal_unwrap -A clippy::useless_vec"
+CLIPPY --tests "-A clippy::bool_assert_comparison -A clippy::assertions_on_constants -A clippy::needless-late-init -A clippy::field_reassign_with_default -A clippy::unnecessary_literal_unwrap"