bip: move the BIP relay into pySim.bip

Move the code from pySim-smpp2sim.py to its own file, so it can be properly
extended.
The current file parses argv, opens a reader and starts the Twisted
reactor at import time, so nothing else can import it.

No functional changes yet, improvements follow in later commits.

Change-Id: Ifd8a15684939977d29ea83a6b669daee14484e88

tests/unittests/test_bip_relay.py

@@ -0,0 +1,45 @@
+#!/usr/bin/env python3
+
+# (C) 2026 by sysmocom - s.f.m.c. GmbH <info@sysmocom.de>
+#
+# Author: Eric Wild
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program 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 General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+import unittest
+
+from pySim.bip import Proact
+from pySim.sms import SMS_SUBMIT, AddressField
+
+
+class BipSinkTest(unittest.TestCase):
+    """Both sinks are optional, a driver with no SMS path at all must not crash and burn
+    with a card that sends one, and one that has one must get the PDU."""
+
+    def _submit(self):
+        return SMS_SUBMIT(tp_da=AddressField('12345', 'unknown', 'unknown'),
+                          tp_ud=b'\x01\x02', tp_udl=2, tp_dcs=0xf6)
+
+    def test_sinks_default_to_none(self):
+        p = Proact()
+        self.assertIsNone(p.sms_sink)
+
+    def test_mo_sms_goes_to_the_sink(self):
+        seen = []
+        Proact(sms_sink=seen.append).send_sms_via_smpp(self._submit())
+        self.assertEqual(len(seen), 1)
+
+    def test_no_sms_sink_drops_instead_of_raising(self):
+        with self.assertLogs('pySim.bip', level='INFO'):
+            Proact().send_sms_via_smpp(self._submit())