Core tests: print to stderr instead of stdout

The failures should be written to stderr anyway, but writing the summary
too means it shows when using the watch script, which prints stderr
messages but hides stdout.

core/Tests.fs

@@ -215,13 +215,13 @@ let applyEdit (edit: Edit) (oldLines: Lines) : Lines =
 
 /// Prints details of a test failure with input, expected and actual columns
 let printFailure (test: Test) (actual: Lines) =
-  printfn "Failed: %A" test.fileName
-  printfn "%s %A" test.language test.settings
+  eprintfn "Failed: %A" test.fileName
+  eprintfn "%s %A" test.language test.settings
   let width = test.settings.column
   let colWidth = width + 10
   let print (cols: string[]) =
     let cols = cols |> Array.map (fun c -> c.PadRight(colWidth).Substring(0, colWidth))
-    printfn "%s" (String.Join(" | ", cols))
+    eprintfn "%s" (String.Join(" | ", cols))
 
   let columns = [| test.input; test.expected; actual |]
 
@@ -269,13 +269,13 @@ let main argv =
         Option.fold run (run acc test) maybeReformatTest
 
     | Error (fileName, lines, err) ->
-        printfn "Error: %A" err
-        printfn "%s" fileName
-        printfn "==%s==" "Input"
-        Seq.iter (printfn "%s") lines
+        eprintfn "Error: %A" err
+        eprintfn "%s" fileName
+        eprintfn "==%s==" "Input"
+        Seq.iter (eprintfn "%s") lines
         { acc with errors = acc.errors + 1 }
 
   let init = {passes = 0; failures = 0; errors = 0}
   let results = Native.files |> Seq.collect readSamplesInFile |> Seq.fold processTest init
-  printfn "Passed: %i; Failed: %i; Errored: %i" results.passes results.failures results.errors
+  eprintfn "Passed: %i; Failed: %i; Errored: %i" results.passes results.failures results.errors
   results.failures + results.errors