delete referencing packages which is common for test projects

Author: smdn

tests/test-assm/Directory.Build.props

@@ -0,0 +1,11 @@
+<!--
+SPDX-FileCopyrightText: 2022 smdn <smdn@smdn.jp>
+SPDX-License-Identifier: MIT
+-->
+<Project>
+  <Import Project="$(MSBuildThisFileDirectory)..\Directory.Build.props" />
+
+  <ItemGroup>
+    <PackageReference Remove="*" />
+  </ItemGroup>
+</Project>