Bump xunit.assert.source from 2.3.1 to 2.4.1 in /Rx.NET/Source

Bumps [xunit.assert.source](https://github.com/xunit/xunit) from 2.3.1 to 2.4.1.
- [Release notes](https://github.com/xunit/xunit/releases)
- [Commits](xunit/xunit@2.3.1...2.4.1)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Rx.NET/Source/src/Microsoft.Reactive.Testing/Microsoft.Reactive.Testing.csproj

@@ -11,7 +11,7 @@
 
   <ItemGroup>
     <EmbeddedResource Include="Properties\*.xml" />
-    <PackageReference Include="xunit.assert.source" Version="2.3.1" PrivateAssets="All" /> 
+    <PackageReference Include="xunit.assert.source" Version="2.4.1" PrivateAssets="All" /> 
     <ProjectReference Include="..\System.Reactive\System.Reactive.csproj" />
   </ItemGroup>
 </Project>