Updated the target and test target definitions in the Package file.

Tests/DocCMiddleware/hummingbird_docc_middlewareTests.swift

@@ -0,0 +1,7 @@
+import Testing
+
+@testable import DocCMiddleware
+
+@Test func example() async throws {
+    // Write your test here and use APIs like `#expect(...)` to check expected conditions.
+}