eng(tests): add NuGetAuditMode=all to tests/Directory.Build.props

Mirror the NuGetAudit setting from src/Directory.Build.props so that
transitive dependencies in test projects are also audited for known CVEs
during dotnet restore.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

tests/Directory.Build.props

@@ -11,6 +11,9 @@
     <Nullable>enable</Nullable>
     <IsPackable>false</IsPackable>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+    <!-- Audit direct + transitive dependencies for known CVEs during restore,
+         matching the setting in src/Directory.Build.props. -->
+    <NuGetAuditMode>all</NuGetAuditMode>
   </PropertyGroup>
 
   <ItemGroup>