Get symbols of DiaSymReader from externals (#2560)

Author: nohwnd

scripts/build/TestPlatform.Dependencies.props

@@ -31,7 +31,7 @@
     <NuGetFrameworksVersion>5.0.0</NuGetFrameworksVersion>
     <JsonNetVersion>9.0.1</JsonNetVersion>
     <MoqVersion>4.7.63</MoqVersion>
-    <TestPlatformExternalsVersion>16.8.0-preview-3968212</TestPlatformExternalsVersion>
+    <TestPlatformExternalsVersion>16.8.0-preview-4040788</TestPlatformExternalsVersion>
     <MicrosoftFakesVersion>16.8.0-beta.20420.2</MicrosoftFakesVersion>
 
     <MicrosoftBuildPackageVersion>16.0.461</MicrosoftBuildPackageVersion>