fix dependencies of AssociatedObjectTests

Author: p-x9

Package.swift

@@ -59,6 +59,7 @@ let package = Package(
             name: "AssociatedObjectTests",
             dependencies: [
                 "AssociatedObject",
+                "AssociatedObjectPlugin",
                 .product(name: "SwiftSyntax", package: "swift-syntax"),
                 .product(name: "SwiftSyntaxMacros", package: "swift-syntax"),
                 .product(name: "SwiftCompilerPlugin", package: "swift-syntax"),