Allow parallel tests to set their own mocks

By storing the Current instance in AsyncLocal, it can be
properly set per-thread and propagated automatically across
async calls.

NOTE: since AsyncLocal<T> only exists in netstandard2.0, I
just removed netstandard1.0 ;). Of course, this is just to
show how we could do this, but if we support this for real,
we'd need to have different implementations for ns1.0 vs
ns2.0.

Caboodle/Caboodle.csproj

@@ -2,8 +2,8 @@
   <PropertyGroup>
     <!--Work around so the conditions work below-->
     <TargetFrameworks></TargetFrameworks>
-    <TargetFrameworks Condition=" '$(OS)' == 'Windows_NT' ">netstandard1.0;netstandard2.0;Xamarin.iOS10;MonoAndroid80;uap10.0.16299</TargetFrameworks>
-    <TargetFrameworks Condition=" '$(OS)' != 'Windows_NT' ">netstandard1.0;netstandard2.0;Xamarin.iOS10;MonoAndroid80</TargetFrameworks>
+    <TargetFrameworks Condition=" '$(OS)' == 'Windows_NT' ">netstandard2.0;net461;Xamarin.iOS10;MonoAndroid80;uap10.0.16299</TargetFrameworks>
+    <TargetFrameworks Condition=" '$(OS)' != 'Windows_NT' ">netstandard2.0;net461;Xamarin.iOS10;MonoAndroid80</TargetFrameworks>
     <AssemblyName>Microsoft.Caboodle</AssemblyName>
     <RootNamespace>Microsoft.Caboodle</RootNamespace>
     <PackageId>Microsoft.Caboodle</PackageId>
@@ -42,8 +42,12 @@
   <ItemGroup>
     <PackageReference Include="mdoc" Version="5.5.0" PrivateAssets="All" />
     <PackageReference Include="MSBuild.Sdk.Extras" Version="1.2.0" PrivateAssets="All" />
+
     <Compile Include="**\*.shared.cs" />
   </ItemGroup>
+  <ItemGroup Condition=" '$(TargetFrameworkIdentifier)' == '.NETFramework' ">
+    <Compile Include="**\*.net.cs" />
+  </ItemGroup>
   <ItemGroup Condition=" $(TargetFramework.StartsWith('netstandard')) ">
     <Compile Include="**\*.netstandard.cs" />
   </ItemGroup>

Caboodle/Geocoding/Geocoding.netstandard.cs

@@ -1,11 +1,18 @@
 using System.Collections.Generic;
+using System.Threading;
 using System.Threading.Tasks;
 
 namespace Microsoft.Caboodle
 {
     public partial class Geocoding
     {
-        public static IGeocoding Current { get; set; }
+        static AsyncLocal<IGeocoding> current;
+
+        public static IGeocoding Current
+        {
+            get => current.Value;
+            set => current.Value = value;
+        }
 
         public static Task<IEnumerable<Placemark>> GetPlacemarksAsync(double latitude, double longitude) =>
             Current?.GetPlacemarksAsync(latitude, longitude) ?? throw new NotImplentedInReferenceAssembly();