[antlir2][tests] add idmap test to basic rust test

Summary:
As I start to enable more rootless test execution, it's important to exercise
the tests-for-the-tests that we have here.
This diff makes sure that in the test container `root` is id `0` and `antlir`
is `1000`, as set in the `antlir` uidmap.

Test Plan:
```
❯ buck2 targets fbcode//antlir/antlir2/testing/tests: | grep rootless | xargs buck2 test
Buck UI: https://www.internalfb.com/buck2/0f869c2e-df9e-4a80-8956-813d1c69ad71
Test UI: https://www.internalfb.com/intern/testinfra/testrun/10133099228309166
Tests finished: Pass 52. Fail 0. Fatal 0. Skip 0. Build failure 0
```

Reviewed By: justintrudell

Differential Revision: D68905617

fbshipit-source-id: 36872d9c60b5c3ada03f00defbb8daa17fa1aa75

antlir/antlir2/testing/tests/BUCK

@@ -24,6 +24,23 @@ image.layer(
             "slow-unit.service",
             target = "default.target",
         ),
+        feature.user_add(
+            home_dir = "/",
+            primary_group = "antlir",
+            shell = "/bin/bash",
+            uidmap = "antlir",
+            username = "antlir",
+        ),
+        feature.group_add(
+            groupname = "antlir",
+            uidmap = "antlir",
+        ),
+        feature.install_text(
+            dst = "/antlir.txt",
+            group = "antlir",
+            text = "I am antlir\n",
+            user = "antlir",
+        ),
     ],
 )
 

antlir/antlir2/testing/tests/test.rs

@@ -5,6 +5,7 @@
  * LICENSE file in the root directory of this source tree.
  */
 
+use std::os::unix::fs::MetadataExt;
 use std::path::Path;
 
 use nix::unistd::getuid;
@@ -55,3 +56,14 @@ fn tmpfs_tmp() {
 fn tmpfs_run() {
     test_tmpfs("/run");
 }
+
+#[test]
+fn id_mapping() {
+    let meta = std::fs::metadata("/").expect("failed to stat /");
+    assert_eq!(0, meta.uid());
+    assert_eq!(0, meta.gid());
+
+    let meta = std::fs::metadata("/antlir.txt").expect("failed to stat /antlir.txt");
+    assert_eq!(1000, meta.uid());
+    assert_eq!(1000, meta.gid());
+}