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

The most complicated code dealing with `Stage2` is the selection of
which types of stdin/stderr to pass to stages, and that's also the
main advantage of the `Stage2` interface. So add a bunch of tests that
the correct types (especially, `io.Pipe()` vs. `os.Pipe()`) are indeed
being selected.

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
+	}
+}