test: Add test for issue-14560

Author: linyihai

tests/testsuite/config.rs

@@ -299,6 +299,29 @@ f1 = 1
     assert_e2e().eq(&output, expected);
 }
 
+#[cargo_test]
+fn config_works_without_extension_run_with_cargo() {
+    write_config_extless(
+        "\
+[foo]
+f1 = 1
+",
+    );
+    let cargo_home = paths::root().join(".cargo");
+    let p = project().file("src/lib.rs", "").build();
+
+    p.cargo("-vV")
+        .env("CARGO_HOME", cargo_home)
+        .with_stderr_data(str![[r#"
+[WARNING] `[ROOT]/.cargo/config` is deprecated in favor of `config.toml`
+[NOTE] if you need to support cargo 1.38 or earlier, you can symlink `config` to `config.toml`
+[WARNING] `[ROOT]/.cargo/config` is deprecated in favor of `config.toml`
+[NOTE] if you need to support cargo 1.38 or earlier, you can symlink `config` to `config.toml`
+
+"#]])
+        .run();
+}
+
 #[cargo_test]
 fn config_ambiguous_filename_symlink_doesnt_warn() {
     // Windows requires special permissions to create symlinks.