Add tests for bitvec_write_field()

This function is actively used by OsmoPCU but have not been covered by
tests so far. The test code is based on
Minh-Quang Nguyen <minh-quang.nguyen@nutaq.com> submission with some
modifications.

The test's FIXME will be addressed in follow-up patches.

Change-Id: I2ee544256b8675bc62a42493aab66a8eeee54f90
Related: OS#1526
diff --git a/tests/bits/bitfield_test.ok b/tests/bits/bitfield_test.ok
new file mode 100644
index 0000000..5cb2500
--- /dev/null
+++ b/tests/bits/bitfield_test.ok
@@ -0,0 +1,36 @@
+=== start test_bitvec_ia_octet_encode_pkt_dl_ass(0) ===
+Encoded PKT DL ASS IA Rest Octets: dd ea db ee f8 00 22 31 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 
+=== end test_bitvec_ia_octet_encode_pkt_dl_ass(0) ===
+
+=== start test_bitvec_ia_octet_encode_pkt_dl_ass(1) ===
+Encoded PKT DL ASS IA Rest Octets: dd ea db ee f8 00 22 31 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 
+=== end test_bitvec_ia_octet_encode_pkt_dl_ass(1) ===
+
+=== start test_bitdiff(22) ===
+=== total 0 bits differ ===
+=== end test_bitdiff(22) ===
+
+=== start test_bitvec_ia_octet_encode_pkt_ul_ass(0) ===
+Encoded PKT UL ASS IA Rest Octets: c0 08 0a 63 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 
+=== end test_bitvec_ia_octet_encode_pkt_ul_ass(0) ===
+
+=== start test_bitvec_ia_octet_encode_pkt_ul_ass(1) ===
+Encoded PKT UL ASS IA Rest Octets: c0 08 0a 63 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 
+=== end test_bitvec_ia_octet_encode_pkt_ul_ass(1) ===
+
+=== start test_bitdiff(22) ===
+=== total 0 bits differ ===
+=== end test_bitdiff(22) ===
+
+=== start test_bitvec_ia_octet_encode_pkt_ul_ass(0) ===
+Encoded PKT UL ASS IA Rest Octets: c8 02 10 0b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 
+=== end test_bitvec_ia_octet_encode_pkt_ul_ass(0) ===
+
+=== start test_bitvec_ia_octet_encode_pkt_ul_ass(1) ===
+Encoded PKT UL ASS IA Rest Octets: c8 02 10 0b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 
+=== end test_bitvec_ia_octet_encode_pkt_ul_ass(1) ===
+
+=== start test_bitdiff(22) ===
+=== total 0 bits differ ===
+=== end test_bitdiff(22) ===
+