If you have VS2008 SP1 installed on your build machine, you can simply add the following property to your TFSBuild.proj file:
<TreatTestFailureAsBuildFailure>true</TreatTestFailureAsBuildFailure>
1 (SP1), , Dev Lead TFS Build, Aaaron Hallberg:
<Target Name="AfterTest">
<GetBuildProperties TeamFoundationServerUrl="$(TeamFoundationServerUrl)"
BuildUri="$(BuildUri)"
Condition=" '$(IsDesktopBuild)' != 'true' ">
<Output TaskParameter="TestSuccess" PropertyName="TestSuccess" />
</GetBuildProperties>
<SetBuildProperties TeamFoundationServerUrl="$(TeamFoundationServerUrl)"
BuildUri="$(BuildUri)"
CompilationStatus="Failed"
Condition=" '$(IsDesktopBuild)' != 'true' and '$(TestSuccess)' != 'true' ">
</Target>