build(deps): update ReactiveUI.Testing requirement in /src

Updates the requirements on [ReactiveUI.Testing](https://github.com/reactiveui/reactiveui) to permit the latest version.
- [Release notes](https://github.com/reactiveui/reactiveui/releases)
- [Commits](reactiveui/ReactiveUI@14.1.1...15.1.1)

---
updated-dependencies:
- dependency-name: ReactiveUI.Testing
  dependency-type: direct:production
...

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

src/Fusillade.Tests/Fusillade.Tests.csproj

@@ -15,6 +15,6 @@
     <PackageReference Include="akavache" Version="7.*" />
     <PackageReference Include="DynamicData" Version="7.*" />
     <PackageReference Include="ReactiveUI" Version="14.*" />
-    <PackageReference Include="ReactiveUI.Testing" Version="14.*" />
+    <PackageReference Include="ReactiveUI.Testing" Version="15.*" />
   </ItemGroup>
 </Project>