tests: Split RFC 2616 date parsing code out and add tests

This makes it testable, and increases its test coverage too 100% of
lines, as measured by `make coverage`.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

src/libostree/ostree-date-utils-private.h

@@ -0,0 +1,38 @@
+/*
+ * Copyright © 2020 Endless OS Foundation LLC
+ *
+ * SPDX-License-Identifier: LGPL-2.0+
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ *
+ * Authors:
+ *  - Philip Withnall <pwithnall@endlessos.org>
+ */
+
+#pragma once
+
+#ifndef __GI_SCANNER__
+
+#include <glib.h>
+
+G_BEGIN_DECLS
+
+GDateTime *_ostree_parse_rfc2616_date_time (const char *buf,
+                                            size_t      len);
+
+G_END_DECLS
+
+#endif