Introduce initial unit test for db_sms_* API

Since OsmoMSC has built-in SMSC, it needs to store the messages
somewhere. Currently we use libdbi and SQLite3 back-end for that.

For a long time, the db_sms_* API remained uncovered by unit tests.
This change aims to fix that, and does cover the following calls:

  - db_sms_store(),
  - db_sms_get(),

  - db_sms_get_next_unsent(),
  - db_sms_mark_delivered(),

  - db_sms_delete_sent_message_by_id(),
  - db_sms_delete_by_msisdn(),
  - db_sms_delete_oldest_expired_message().

Due to performance reasons, the test database is initialized in
RAM using the magic filename ':memory:'. This is a feature of
SQLite3 (and not libdbi), see:

  https://www.sqlite.org/inmemorydb.html

Of course, this unit test helped to discover some problems:

  1) Storing an SMS with empty TP-User-Data (TP-UDL=0) causes
     buffer overruns in both db_sms_store() and db_sms_get().

  2) TP-User-Data-Length is always being interpreted in octets,
     regardless of DCS (Data Coding Scheme). This results in
     storing garbage in the database if the default 7-bit
     encoding is used. Fortunately, the 'user_data' buffer
     in structure 'gsm_sms' is large emough, so we don't
     experience buffer overruns.

  3) db_sms_delete_oldest_expired_message() doesn't work
     as expected. Instead of removing the *oldest* expired
     message, it tries to remove the *newest* one.

The current test expectations do reflect these problems.
All of them will be fixed in the follow-up patches.

Change-Id: Id94ad35b6f78f839137db2e17010fbf9b40111a3

configure.ac

@@ -251,6 +251,7 @@ AC_OUTPUT(
     tests/Makefile
     tests/atlocal
     tests/smpp/Makefile
+    tests/db_sms/Makefile
     tests/sms_queue/Makefile
     tests/msc_vlr/Makefile
     doc/Makefile