test(package): add test showing target dir not excluded from backups

Add a failing test that demonstrates the current buggy behavior where
`cargo package` does not create a CACHEDIR.TAG file in the target
directory. This causes backup tools to unnecessarily include temporary
packaging artifacts.

The test currently passes by asserting that CACHEDIR.TAG does NOT
exist (the buggy behavior). The next commit will implement the fix
and update this test to verify correct behavior.

Relates to #16238

Author: SepehrGanji

tests/testsuite/package.rs

@@ -7804,3 +7804,31 @@ fn publish_to_alt_registry_warns() {
 "#]])
         .run();
 }
+
+#[cargo_test]
+fn package_dir_not_excluded_from_backups() {
+    // This test documents the current behavior where target directory is NOT excluded from backups.
+    // After the fix, this test will be updated to verify that CACHEDIR.TAG exists.
+    let p = project().file("src/lib.rs", "").build();
+
+    p.cargo("package --allow-dirty")
+        .with_stderr_data(str![[r#"
+[WARNING] manifest has no description, license, license-file, documentation, homepage or repository
+  |
+  = [NOTE] see https://doc.rust-lang.org/cargo/reference/manifest.html#package-metadata for more info
+[PACKAGING] foo v0.0.1 ([ROOT]/foo)
+[PACKAGED] 4 files, [FILE_SIZE]B ([FILE_SIZE]B compressed)
+[VERIFYING] foo v0.0.1 ([ROOT]/foo)
+[COMPILING] foo v0.0.1 ([ROOT]/foo/target/package/foo-0.0.1)
+[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
+
+"#]])
+        .run();
+
+    // Verify CACHEDIR.TAG does NOT exist in target (documenting the buggy behavior)
+    let cachedir_tag = p.root().join("target/CACHEDIR.TAG");
+    assert!(
+        !cachedir_tag.exists(),
+        "CACHEDIR.TAG should not exist yet (this is the bug)"
+    );
+}