If you have VS2008 SP1 installed on your build machine then you can simply add the following property to your TFSBuild.proj file:
<TreatTestFailureAsBuildFailure>true</TreatTestFailureAsBuildFailure>
If you don't have SP1 installed and you don't want to install it, then you can do it the old fashioned route as detailed here by the Dev Lead on the TFS Build team, Aaaron Hallberg:
<Target Name="AfterTest">
<!-- Refresh the build properties. -->
<GetBuildProperties TeamFoundationServerUrl="$(TeamFoundationServerUrl)"
BuildUri="$(BuildUri)"
Condition=" '$(IsDesktopBuild)' != 'true' ">
<Output TaskParameter="TestSuccess" PropertyName="TestSuccess" />
</GetBuildProperties>
<!-- Set CompilationStatus to Failed if TestSuccess is false. -->
<SetBuildProperties TeamFoundationServerUrl="$(TeamFoundationServerUrl)"
BuildUri="$(BuildUri)"
CompilationStatus="Failed"
Condition=" '$(IsDesktopBuild)' != 'true' and '$(TestSuccess)' != 'true' ">
</Target>