Allow extracting koji event from another compose

When multiple composes are chained, they should reuse the same event.
However it is tricky as the value would have to be passed by hand. This
patch makes it possible to read the value from another compose (the
first one in the chain).

JIRA: COMPOSE-2571
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

tests/test_util.py

@@ -1,6 +1,7 @@
 #!/usr/bin/env python2
 # -*- coding: utf-8 -*-
 
+import argparse
 import mock
 import os
 import sys
@@ -685,5 +686,23 @@ class TestTZOffset(unittest.TestCase):
         self.assertEqual(util.get_tz_offset(), "+00:00")
 
 
+class TestParseKojiEvent(PungiTestCase):
+
+    def test_number(self):
+        self.assertEqual(util.parse_koji_event("1234"), 1234)
+
+    def test_correct_path(self):
+        touch(
+            os.path.join(self.topdir, 'work/global/koji-event'),
+            '{"id": 19769058, "ts": 1527641311.22855}'
+        )
+
+        self.assertEqual(util.parse_koji_event(self.topdir), 19769058)
+
+    def test_bad_path(self):
+        with self.assertRaises(argparse.ArgumentTypeError):
+            util.parse_koji_event(self.topdir)
+
+
 if __name__ == "__main__":
     unittest.main()