Add two xunit packages to runtime.depproj to unblock about 90 tests (#6607)

* Add two xunit packages to runtime.depproj to unblock about 90 tests

This change adds the packages "xunit.core" and "xunit.performance.core"
required by some CoreCLR tests to external test dependencies. I have
copied the clauses from external.depproj in CoreCLR and their versions
from dependency.props. This change reduces the number of failures from
346 to 254.

Thanks

Tomas

tests/external/runtime/runtime.depproj

@@ -21,6 +21,12 @@
     <PackageReference Include="System.Runtime.CompilerServices.Unsafe">
       <Version>4.6.0-preview.18571.1</Version>
     </PackageReference>
+    <PackageReference Include="xunit.core">
+      <Version>2.2.0-beta2-build3300</Version>
+    </PackageReference>
+    <PackageReference Include="xunit.performance.core">
+      <Version>1.0.0-beta-build0015</Version>
+    </PackageReference>
     
   </ItemGroup>