Add helper functions for AMR codec

* add functions to encode/decode various codec paramters from RTP payload with
  AMR frame according to RFC 4867
* those functions are extended version based on code from osmo-bts'
  amr.c by Andreas Eversberg
* add corresponding enum types and strings for logging
* add regression tests

It's useful both to replace manual parsing in osmo-bts with fuctions
covered by test suite and as a debugging helpers for issues related to
AMR.

Change-Id: Ia217679a07d3fbc970f435e20f6eac33d34bd597
Related: OS#1562
Reviewed-on: https://gerrit.osmocom.org/118
Tested-by: Jenkins Builder
Reviewed-by: Holger Freyther <holger@freyther.de>
diff --git a/tests/testsuite.at b/tests/testsuite.at
index aa269af..d49f7ff 100644
--- a/tests/testsuite.at
+++ b/tests/testsuite.at
@@ -115,12 +115,17 @@
 AT_CHECK([$abs_top_builddir/tests/logging/logging_test], [0], [expout], [experr])
 AT_CLEANUP
 
+AT_SETUP([codec])
+AT_KEYWORDS([codec])
+cat $abs_srcdir/codec/codec_test.ok > expout
+AT_CHECK([$abs_top_builddir/tests/codec/codec_test], [0], [expout], [ignore])
+AT_CLEANUP
+
 AT_SETUP([fr])
 AT_KEYWORDS([fr])
 cat $abs_srcdir/fr/fr_test.ok > expout
 cat $abs_srcdir/fr/fr_test.err > experr
 AT_CHECK([$abs_top_builddir/tests/fr/fr_test], [0], [expout], [experr])
-
 AT_CLEANUP
 
 AT_SETUP([loggingrb])