Add initial version of hints tests, to show pre-[hints] Cargo behavior

These tests show what prior versions of Cargo will do with hints.
The subsequent addition of support for hints will modify these tests to
reflect the corresponding changes to Cargo.

tests/testsuite/hints.rs

@@ -0,0 +1,59 @@
+//! Tests for hints.
+
+use crate::prelude::*;
+use cargo_test_support::{project, str};
+
+#[cargo_test]
+fn empty_hints_warn() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+            [package]
+            name = "foo"
+            version = "0.0.1"
+            edition = "2015"
+
+            [hints]
+            "#,
+        )
+        .file("src/main.rs", "fn main() {}")
+        .build();
+    p.cargo("check -v")
+        .with_stderr_data(str![[r#"
+[WARNING] unused manifest key: hints
+[CHECKING] foo v0.0.1 ([ROOT]/foo)
+[RUNNING] `rustc --crate-name foo [..]`
+[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
+
+"#]])
+        .run();
+}
+
+#[cargo_test]
+fn unknown_hints_warn() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+            [package]
+            name = "foo"
+            version = "0.0.1"
+            edition = "2015"
+
+            [hints]
+            this-is-an-unknown-hint = true
+            "#,
+        )
+        .file("src/main.rs", "fn main() {}")
+        .build();
+    p.cargo("check -v")
+        .with_stderr_data(str![[r#"
+[WARNING] unused manifest key: hints
+[CHECKING] foo v0.0.1 ([ROOT]/foo)
+[RUNNING] `rustc --crate-name foo [..]`
+[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
+
+"#]])
+        .run();
+}

tests/testsuite/main.rs

@@ -106,6 +106,7 @@ mod git_shallow;
 mod glob_targets;
 mod global_cache_tracker;
 mod help;
+mod hints;
 mod https;
 mod inheritable_workspace_fields;
 mod install;