test: add in-process fake ElectrumX server for network-layer testing

Real loopback TCP socket speaking newline-delimited JSON-RPC 2.0, optionally
TLS with a self-signed certificate, with per-method handlers and call
counters. Lets ElectrumClient, WalletSynchronizer, TransactionInspector, and
CertificatePinStore be exercised against the same code paths used in
production instead of being mocked.

Also exposes UpdateChecker's tag-parsing logic internally so its version
comparison can be tested without a real GitHub API call.

src/Core/PalladiumWallet.Core.csproj

@@ -6,8 +6,12 @@
     <Nullable>enable</Nullable>
   </PropertyGroup>
 
-  <ItemGroup>
-    <PackageReference Include="NBitcoin" Version="10.0.6" />
+  <ItemGroup>
+    <PackageReference Include="NBitcoin" Version="10.0.6" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <InternalsVisibleTo Include="PalladiumWallet.Tests" />
   </ItemGroup>
 
 </Project>