Add basic tests for completion-tool list-{dependencies,snippets} (#8065)

There were not tests covering off these two completion-tool commands.
This covers off the happy path for all the `list-*` commands. There are
still not tests for the completion-tool `generate-*-script` commands

Author: plemarquand

Fixtures/DependencyResolution/External/Complex/deck-of-playing-cards-local/Package.swift

@@ -0,0 +1,19 @@
+// swift-tools-version:4.2
+import PackageDescription
+
+let package = Package(
+    name: "DeckOfPlayingCards",
+    products: [
+        .library(name: "DeckOfPlayingCards", targets: ["DeckOfPlayingCards"]),
+    ],
+    dependencies: [
+        .package(path: "../PlayingCard"),
+        .package(path: "../FisherYates")
+    ],
+    targets: [
+        .target(
+            name: "DeckOfPlayingCards",
+            dependencies: ["PlayingCard", "FisherYates"],
+            path: "src"),
+    ]
+)

Fixtures/DependencyResolution/External/Complex/deck-of-playing-cards-local/src/Deck.swift

@@ -551,6 +551,20 @@ final class PackageCommandTests: CommandsTestCase {
         }
     }
 
+    func testCompletionToolListSnippets() async throws {
+        try await fixture(name: "Miscellaneous/Plugins/PluginsAndSnippets") { fixturePath in
+            let result = try await execute(["completion-tool", "list-snippets"], packagePath: fixturePath)
+            XCTAssertEqual(result.stdout, "MySnippet\n")
+        }
+    }
+
+    func testCompletionToolListDependencies() async throws {
+        try await fixture(name: "DependencyResolution/External/Complex") { fixturePath in
+            let result = try await execute(["completion-tool", "list-dependencies"], packagePath: fixturePath.appending("deck-of-playing-cards-local"))
+            XCTAssertEqual(result.stdout, "playingcard\nfisheryates\n")
+        }
+    }
+
     func testCompletionToolListExecutables() async throws {
         try await fixture(name: "Miscellaneous/MultipleExecutables") { fixturePath in
             let result = try await execute(["completion-tool", "list-executables"], packagePath: fixturePath)