Fix FSharp tests

[chrisjhoare]

Possible fix for #3403

This adds fsharp tests to the build and fixes two failing tests. I'm not entirely sure about the full context around changing the ToExpression overload however.

[Aaronontheweb]

Hmmm, looks like XUnit had an issue when trying to execute these tests:

[16:26:41] :		 [RunTests] 	D:\work\edbce350daf6a08d\src\core\Akka.Tests\Akka.Tests.csproj
[16:26:41] :		 [RunTests] dotnet xunit -f net452 -c Release -nobuild -parallel none -teamcity -xml D:\work\edbce350daf6a08d\TestResults\Akka.Tests_xunit.xml
[16:26:41] :		 [RunTests] Detecting target frameworks in Akka.Tests.csproj...
[16:26:42] :		 [RunTests] Locating binaries for framework net452...
[16:26:42] :		 [RunTests] Running desktop CLR tests for framework net452...
[16:26:42] :		 [RunTests] xUnit.net Console Runner (64-bit Desktop .NET 4.0.30319.42000)
[16:26:43] :		 [RunTests] System.IO.FileLoadException: Could not load file or assembly 'FSharp.Core, Version=4.4.3.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. Strong name signature could not be verified.  The assembly may have been tampered with, or it was delay signed but not fully signed with the correct private key. (Exception from HRESULT: 0x80131045)
[16:26:43] :		 [RunTests] File name: 'FSharp.Core, Version=4.4.3.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' ---> System.IO.FileLoadException: Could not load file or assembly 'FSharp.Core, Version=4.4.1.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. Strong name signature could not be verified.  The assembly may have been tampered with, or it was delay signed but not fully signed with the correct private key. (Exception from HRESULT: 0x80131045)
[16:26:43] :		 [RunTests] File name: 'FSharp.Core, Version=4.4.1.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'
[16:26:43] :		 [RunTests] 

[Aaronontheweb]

Strong name signature huh.... That's a new one.