Add test harness and run the first program (#5)

WoofWare.PawPrint.Test/LoggerFactory.fs

@@ -0,0 +1,64 @@
+namespace WoofWare.PawPrint.Test
+
+open System
+open Microsoft.Extensions.Logging
+
+type LogLine =
+    {
+        Level : LogLevel
+        LoggerName : string
+        Message : string
+    }
+
+/// Very small, in-memory implementation of `ILoggerFactory` for unit tests.
+[<RequireQualifiedAccess>]
+module LoggerFactory =
+
+    /// Returns a pair: `(getLogs, loggerFactory)` where `getLogs ()` retrieves all
+    /// log messages emitted so far, in chronological order.
+    let makeTest () : (unit -> LogLine list) * ILoggerFactory =
+        // Shared sink for all loggers created by the factory.
+        let sink = ResizeArray ()
+
+        let createLogger (category : string) : ILogger =
+            { new ILogger with
+                member _.BeginScope _state =
+                    { new IDisposable with
+                        member _.Dispose () = ()
+                    }
+
+                member _.IsEnabled _logLevel = true
+
+                member _.Log (logLevel, eventId, state, ex, formatter) =
+                    let message =
+                        try
+                            formatter.Invoke (state, ex)
+                        with _ ->
+                            "<formatter threw>"
+
+                    lock
+                        sink
+                        (fun () ->
+                            {
+                                Level = logLevel
+                                LoggerName = category
+                                Message = message
+                            }
+                            |> sink.Add
+                        )
+            }
+
+        // Minimal `ILoggerFactory` that just hands out instances of the above.
+        let factory =
+            { new ILoggerFactory with
+                member _.CreateLogger categoryName = createLogger categoryName
+
+                member _.AddProvider _provider = ()
+
+                member _.Dispose () = ()
+            }
+
+        // Expose accessor that snapshots the current sink contents.
+        let getLogs () = lock sink (fun () -> List.ofSeq sink)
+
+        getLogs, factory