Bump Microsoft.Reactive.Testing from 4.1.4 to 4.1.5 in /exercises
Bump Microsoft.Reactive.Testing from 4.1.4 to 4.1.5 in /exercises

Bumps [Microsoft.Reactive.Testing](https://github.com/dotnet/reactive) from 4.1.4 to 4.1.5.
- [Release notes](https://github.com/dotnet/reactive/releases)
- [Commits](https://github.com/dotnet/reactive/compare/rxnet-v4.1.4...rxnet-v4.1.5)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
exercism/xcsharp,robkeim/xcsharp,exercism/xcsharp,robkeim/xcsharp