Add basic Cargo snapshot test

Author: Kobzol

src/bootstrap/src/core/build_steps/tool.rs

@@ -802,6 +802,8 @@ impl Step for Rustdoc {
     }
 }
 
+/// Builds the cargo tool.
+/// Note that it can be built using a stable compiler.
 #[derive(Debug, Clone, Hash, PartialEq, Eq)]
 pub struct Cargo {
     pub compiler: Compiler,

src/bootstrap/src/core/builder/tests.rs

@@ -1000,6 +1000,19 @@ mod snapshot {
         ");
     }
 
+    #[test]
+    fn build_cargo() {
+        let ctx = TestCtx::new();
+        insta::assert_snapshot!(
+            ctx.config("build")
+                .paths(&["cargo"])
+            .render_steps(), @r"
+        [build] llvm <host>
+        [build] rustc 0 <host> -> rustc 1 <host>
+        [build] rustc 1 <host> -> cargo 2 <host>
+        ");
+    }
+
     #[test]
     fn dist_default_stage() {
         let ctx = TestCtx::new();