Add AsMut to minicore prelude::v1

Author: A4-Tacks

src/tools/rust-analyzer/crates/test-utils/src/minicore.rs

@@ -1941,6 +1941,7 @@ pub mod prelude {
             clone::Clone,                            // :clone
             cmp::{Eq, PartialEq},                    // :eq
             cmp::{Ord, PartialOrd},                  // :ord
+            convert::AsMut,                          // :as_mut
             convert::AsRef,                          // :as_ref
             convert::{From, Into, TryFrom, TryInto}, // :from
             default::Default,                        // :default