test: Add test codebase for shell completions

tests/testsuite/shell_completions/mod.rs

@@ -1,4 +1,22 @@
-#![allow(unused)]
+use std::path::PathBuf;
+
+use cargo::util::command_prelude::*;
+use cargo_test_support::cargo_test;
+
+#[cargo_test]
+fn test_get_registry_candidates() {
+    let current_dir = std::env::current_dir().expect("Failed to get current directory");
+    let cwd = PathBuf::from(file!()).parent().unwrap().join("template");
+    let cwd = current_dir.join(cwd);
+
+    let expected = snapbox::str![
+        "my-registry1
+my-registry2"
+    ];
+    let actual = print_candidates(get_registry_candidates(Some(cwd)).unwrap());
+    snapbox::assert_data_eq!(actual, expected);
+}
+
 fn print_candidates(candidates: Vec<clap_complete::CompletionCandidate>) -> String {
     candidates
         .into_iter()

tests/testsuite/shell_completions/template/.cargo/config.toml

@@ -0,0 +1,3 @@
+[registries]
+my-registry1 = { index = "my-registry1"}
+my-registry2 = { index = "my-registry2"}