Backend.Tests: add missing packages.config ref

While Fsdk is referenced in fsproj file it was not present
in packages.config, it did not cause red CI because Fsdk
was referenced in another packages.config file.

src/GWallet.Backend.Tests/packages.config

@@ -1,6 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <packages>
   <package id="FSharp.Core" version="4.7.0" targetFramework="net461" />
+  <package id="Fsdk" version="0.6.0--date20230530-1155.git-3bb8d08" targetFramework="net46" />
   <package id="Microsoft.Extensions.Logging.Abstractions" version="1.0.2" targetFramework="net46" />
   <package id="NBitcoin" version="6.0.17" targetFramework="net471" />
   <package id="NBitcoin.Altcoins" version="3.0.8" targetFramework="net452" />