Bump the test-dependencies group with 3 updates

Bumps Microsoft.NET.Test.Sdk from 17.11.1 to 18.6.0
Bumps xunit from 2.9.2 to 2.9.3
Bumps xunit.runner.visualstudio from 2.8.2 to 3.1.5

---
updated-dependencies:
- dependency-name: Microsoft.NET.Test.Sdk
  dependency-version: 18.6.0
  dependency-type: direct:production
  update-type: version-update:semver-major
  dependency-group: test-dependencies
- dependency-name: xunit
  dependency-version: 2.9.3
  dependency-type: direct:production
  update-type: version-update:semver-patch
  dependency-group: test-dependencies
- dependency-name: xunit.runner.visualstudio
  dependency-version: 3.1.5
  dependency-type: direct:production
  update-type: version-update:semver-major
  dependency-group: test-dependencies
...

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

Author: dependabot[bot]

tests/PtpLabClock.Protocol.Tests/PtpLabClock.Protocol.Tests.csproj

@@ -7,9 +7,9 @@
     <IsPackable>false</IsPackable>
   </PropertyGroup>
   <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.11.1" />
-    <PackageReference Include="xunit" Version="2.9.2" />
-    <PackageReference Include="xunit.runner.visualstudio" Version="2.8.2">
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.6.0" />
+    <PackageReference Include="xunit" Version="2.9.3" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="3.1.5">
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
       <PrivateAssets>all</PrivateAssets>
     </PackageReference>