maildir,notmuch: avoid leaking open files

Previously, Message.NewReader returned the wrapped buffered reader
without a reference to the opened file, so the files descriptors
were left unclosed after reading.  Now, the file reader is returned
directly and closed on the call site.  Buffering is not needed here
because it is an implementation detail of go-message.

Fixes: https://todo.sr.ht/~rjarry/aerc/9

worker/lib/parse_test.go

@@ -1,9 +1,9 @@
 package lib
 
 import (
-	"bytes"
 	"io"
 	"io/ioutil"
+	"os"
 	"path/filepath"
 	"testing"
 
@@ -36,23 +36,17 @@ func TestMessageInfoHandledError(t *testing.T) {
 }
 
 type mockRawMessage struct {
-	body []byte
-}
-
-func newMockRawMessage(body []byte) *mockRawMessage {
-	return &mockRawMessage{
-		body: body,
-	}
+	path string
 }
 
 func newMockRawMessageFromPath(p string) *mockRawMessage {
-	b, err := ioutil.ReadFile(p)
-	die(err)
-	return newMockRawMessage(b)
+	return &mockRawMessage{
+		path: p,
+	}
 }
 
-func (m *mockRawMessage) NewReader() (io.Reader, error) {
-	return bytes.NewReader(m.body), nil
+func (m *mockRawMessage) NewReader() (io.ReadCloser, error) {
+	return os.Open(m.path)
 }
 func (m *mockRawMessage) ModelFlags() ([]models.Flag, error) { return nil, nil }
 func (m *mockRawMessage) Labels() ([]string, error)          { return nil, nil }