Add some tests that Pipeline.Start() picks the right stdin/stdout

The most complicated code dealing with the change to `Stage.Start()`
is the selection of which types of stdin/stderr to pass to stages, and
that's also the main advantage of the new interface. So add a bunch of
tests that the correct types (especially, `io.Pipe()` vs. `os.Pipe()`)
are indeed being selected.

Author: mhagger

pipe/export_test.go

@@ -0,0 +1,4 @@
+package pipe
+
+// This file exports a functions to be used only for testing.
+var UnwrapNopCloser = unwrapNopCloser

pipe/nop_closer.go

@@ -51,3 +51,19 @@ type writerNopCloser struct {
 func (w writerNopCloser) Close() error {
 	return nil
 }
+
+// unwrapNopCloser unwraps the object if it is some kind of nop
+// closer, and returns the underlying object. This function is used
+// only for testing.
+func unwrapNopCloser(obj any) (any, bool) {
+	switch obj := obj.(type) {
+	case readerNopCloser:
+		return obj.Reader, true
+	case readerWriterToNopCloser:
+		return obj.Reader, true
+	case writerNopCloser:
+		return obj.Writer, true
+	default:
+		return nil, false
+	}
+}