build(deps): update ReactiveUI.Testing requirement in /src (#706)

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] <[email protected]>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

src/Akavache.Tests/Akavache.Tests.csproj

@@ -15,7 +15,7 @@
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.3" />
     <PackageReference Include="Xunit.SkippableFact" Version="1.4.13" />
     <PackageReference Include="Xunit.StaFact" Version="1.0.37" />
-    <PackageReference Include="ReactiveUI.Testing" Version="14.*" />
+    <PackageReference Include="ReactiveUI.Testing" Version="15.*" />
     <PackageReference Include="System.Net.Http" Version="4.3.4" />
     <PackageReference Include="SQLitePCLRaw.bundle_e_sqlite3" Version="2.0.4" />
     <PackageReference Include="FluentAssertions" Version="6.0.0" />