Fix break in tests from VS 17.3

This is a workaround suggested in dotnet/msbuild#7873

Author: AArnott

src/NerdBank.GitVersioning.Tests/NerdBank.GitVersioning.Tests.csproj

@@ -36,6 +36,7 @@
     <PackageReference Include="Microsoft.Build" Version="15.1.548" ExcludeAssets="runtime" />
     <PackageReference Include="Microsoft.Build.Utilities.Core" Version="15.1.548" ExcludeAssets="runtime" />
     <PackageReference Include="Microsoft.Build.Locator" Version="1.4.1" />
+    <PackageReference Include="System.Memory" Version="4.5.5" />
     <PackageReference Include="System.Threading.Tasks.Dataflow" Version="6.0.0" />
     <PackageReference Include="Xunit.Combinatorial" Version="1.2.7" />
     <PackageReference Include="xunit" Version="2.4.1" />