Tests: remove @testable import QueryEngine

Use the newly extended APIs for testing and use a non-testable import of
the package, avoiding a recompilation.

Author: compnerd

Tests/QueryEngineTests/QueryEngineTests.swift

@@ -15,7 +15,7 @@ import _AsyncFileSystem
 import Basics
 import Crypto
 import struct Foundation.Data
-@testable import QueryEngine
+import QueryEngine
 import struct SystemPackage.FilePath
 import _InternalTestSupport
 import Testing