test(custom-toolchain): migrate to .expect() API

This change is done to be consistent with the new migration to the
`.expect()` API's that were defined in #4334 and #4342, and are
showcased in the latest PR for migration (#4343).

Author: FranciscoTGouveia

tests/suite/cli_v2.rs

@@ -836,28 +836,34 @@ async fn list_targets_v1_toolchain() {
 
 #[tokio::test]
 async fn list_targets_custom_toolchain() {
-    let mut cx = CliTestContext::new(Scenario::SimpleV2).await;
-    cx.config.expect_ok(&["rustup", "default", "stable"]).await;
+    let cx = CliTestContext::new(Scenario::SimpleV2).await;
+    cx.config
+        .expect(&["rustup", "default", "stable"])
+        .await
+        .is_ok();
     let stable_path = cx
         .config
         .rustupdir
         .join("toolchains")
         .join(format!("stable-{}", this_host_triple()));
     cx.config
-        .expect_ok(&[
+        .expect([
             "rustup",
             "toolchain",
             "link",
             "stuff",
             &stable_path.to_string_lossy(),
         ])
-        .await;
+        .await
+        .is_ok();
     cx.config
-        .expect_stdout_ok(
-            &["rustup", "+stuff", "target", "list", "--installed"],
-            &this_host_triple().to_string(),
-        )
-        .await;
+        .expect(["rustup", "+stuff", "target", "list", "--installed"])
+        .await
+        .with_stdout(snapbox::str![[r#"
+[HOST_TRIPLE]
+
+"#]])
+        .is_ok();
 }
 
 #[tokio::test]