Bump NUnit3TestAdapter from 3.12.0 to 3.15.1

Bumps [NUnit3TestAdapter](https://github.com/nunit/nunit3-vs-adapter) from 3.12.0 to 3.15.1.
- [Release notes](https://github.com/nunit/nunit3-vs-adapter/releases)
- [Commits](nunit/nunit3-vs-adapter@V3.12...V3.15.1)

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

src/Serilog.Enrichers.CorrelationId.Tests/Serilog.Enrichers.CorrelationId.Tests.csproj

@@ -8,7 +8,7 @@
         <PackageReference Include="FakeItEasy" Version="5.4.0" />
         <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.4.0" />
         <PackageReference Include="NUnit" Version="3.12.0" />
-        <PackageReference Include="NUnit3TestAdapter" Version="3.12.0" />
+        <PackageReference Include="NUnit3TestAdapter" Version="3.15.1" />
         <PackageReference Include="Appveyor.TestLogger" Version="2.0.0" />
         <PackageReference Include="coverlet.msbuild" Version="2.7.0" />
         <PackageReference Include="Microsoft.AspNetCore.Http" Version="2.2.2" />