build(deps): bump Microsoft.Reactive.Testing from 4.2.0 to 4.3.1 (#445)

Bumps [Microsoft.Reactive.Testing](https://github.com/dotnet/reactive) from 4.2.0 to 4.3.1.
- [Release notes](https://github.com/dotnet/reactive/releases)
- [Commits](dotnet/reactive@rxnet-v4.2.0...rxnet-v4.3.1)

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

src/Directory.build.props

@@ -40,7 +40,7 @@
     <PackageReference Include="xunit" Version="2.4.1" />
     <PackageReference Include="Xunit.StaFact" Version="0.3.18" />
     <PackageReference Include="Shouldly" Version="4.0.0-beta0002" />
-    <PackageReference Include="Microsoft.Reactive.Testing" Version="4.2.0" />
+    <PackageReference Include="Microsoft.Reactive.Testing" Version="4.3.1" />
     <PackageReference Include="PublicApiGenerator" Version="9.3.0" />
   </ItemGroup>