Bump Akka.TestKit from 1.5.6 to 1.5.7

Bumps [Akka.TestKit](https://github.com/akkadotnet/akka.net) from 1.5.6 to 1.5.7.
- [Release notes](https://github.com/akkadotnet/akka.net/releases)
- [Changelog](https://github.com/akkadotnet/akka.net/blob/dev/RELEASE_NOTES.md)
- [Commits](akkadotnet/akka.net@1.5.6...1.5.7)

---
updated-dependencies:
- dependency-name: Akka.TestKit
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

src/Directory.Build.props

@@ -18,6 +18,6 @@
 	<PropertyGroup>
 		<NUnitVersion>3.13.3</NUnitVersion>
 		<NUnitAdapterVersion>4.4.2</NUnitAdapterVersion>
-		<AkkaTestKitVersion>1.5.6</AkkaTestKitVersion>
+		<AkkaTestKitVersion>1.5.7</AkkaTestKitVersion>
 	</PropertyGroup>
 </Project>