Test capacity of ZST vector

Initially, rust-lang#50233 accidentally changed the capacity of empty ZST. This
was pointed out during code review. This commit adds a test to prevent
capacity of ZST vectors from accidentally changing to prevent that
from happening again.

src/liballoc/tests/vec.rs

@@ -79,6 +79,11 @@ fn test_reserve() {
     assert!(v.capacity() >= 33)
 }
 
+#[test]
+fn test_zst_capacity() {
+    assert_eq!(Vec::<()>::new().capacity(), usize::max_value());
+}
+
 #[test]
 fn test_extend() {
     let mut v = Vec::new();