disable parallelism across assemblies

[SurajGupta]

• By default, NUnit doesn't run tests in parallel within an assembly. Removing AssemblyInfo.cs directives because those aren't (according to documentation) doing anything. Those control the degree of parallelism for tests that opt-into parallelism
• NUnit does run tests across assemblies in parallel. AppVeyor.yml now calls the NUnit CLI directly, setting agents=1 to disable that
• No longer using the "Previous" VS 2015 build image. Now using the "current" one.
• Build still fails on a single unit test. This same unit test consistently fails with different NUnit CLI arguments, so I think it might be a legitimate failure:

1) Error : Microsoft.Its.Domain.Sql.Tests.DiagnosticTests.Sensor_can_be_used_to_check_read_model_catchup_status
System.AggregateException : One or more errors occurred.
  ----> System.InvalidOperationException : The cast to value type 'System.Int64' failed because the materialized value is null. Either the result type's generic parameter or the query must use a nullable type.
   at System.Threading.Tasks.Task`1.GetResultCore(Boolean waitCompletionNotification)
   at Microsoft.Its.Domain.Sql.Tests.DiagnosticTests.Sensor_can_be_used_to_check_read_model_catchup_status() in C:\projects\its-cqrs\Domain.Sql.Tests\DiagnosticTests.cs:line 16
--InvalidOperationException
   at System.Data.Entity.Core.Common.Internal.Materialization.Shaper.ErrorHandlingValueReader`1.GetValue(DbDataReader reader, Int32 ordinal)
   at lambda_method(Closure , Shaper )
   at System.Data.Entity.Core.Common.Internal.Materialization.Coordinator`1.ReadNextElement(Shaper shaper)
   at System.Data.Entity.Core.Common.Internal.Materialization.Shaper`1.SimpleEnumerator.<MoveNextAsync>d__4.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Data.Entity.Internal.LazyAsyncEnumerator`1.<FirstMoveNextAsync>d__0.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Data.Entity.Infrastructure.IDbAsyncEnumerableExtensions.<SingleAsync>d__2d`1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.Its.Domain.Sql.Sensors.<CatchupStatus>d__4.MoveNext() in C:\projects\its-cqrs\Domain.Sql\Sensors.cs:line 42