Prevent tests from importing "normal" NuGet props/targets (#80573)

We import them manually from a shared location.

Usually this is not a problem because batch build (via the build script)
does its own restore thing, but it is a problem in case the project is
restored on its own, such as when it is built with "dotnet build".

src/tests/Directory.Build.props

@@ -196,6 +196,10 @@
     <BuildAsStandalone Condition="'$(BuildAsStandalone)' == ''">true</BuildAsStandalone>
     <OutputType Condition="$(BuildAsStandalone)">Exe</OutputType>
     <TestFramework>GeneratedRunner</TestFramework>
+
+    <!-- Prevent project-specific NuGet props/targets from clashing with the ones we manually import below. -->
+    <ImportProjectExtensionProps>false</ImportProjectExtensionProps>
+    <ImportProjectExtensionTargets>false</ImportProjectExtensionTargets>
   </PropertyGroup>
 
   <Import Project="$(RepositoryEngineeringDir)testing\tests.props"  Condition="'$(IsTestsCommonProject)' != 'true'" />