Bump Microsoft.NET.Test.Sdk from 17.14.1 to 18.0.0 (#326)

Author: vcsjones

Directory.Packages.props

@@ -14,7 +14,7 @@
     <PackageVersion Include="XenoAtom.CommandLine" Version="1.0.1" />
     <PackageVersion Include="MinVer" Version="6.0.0" />
 
-    <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="17.14.1" />
+    <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="18.0.0" />
     <PackageVersion Include="xunit.v3" Version="3.1.0" />
     <PackageVersion Include="xunit.runner.visualstudio" Version="3.1.4" />
   </ItemGroup>