Bump NUnit from 4.3.0 to 4.3.1 in /src (#257)

Author: dependabot[bot]

src/Directory.Packages.props

@@ -7,7 +7,7 @@
     <PackageVersion Include="Codeuctivity.ImageSharpCompare" Version="4.0.298" />
     <PackageVersion Include="MarkdownSnippets.MsBuild" Version="27.0.2" />
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="17.12.0" />
-    <PackageVersion Include="NUnit" Version="4.3.0" />
+    <PackageVersion Include="NUnit" Version="4.3.1" />
     <PackageVersion Include="NUnit3TestAdapter" Version="4.6.0" />
     <PackageVersion Include="Polyfill" Version="7.8.0" />
     <PackageVersion Include="ProjectDefaults" Version="1.0.147" />