GSM 04.08: Add support for parsing CSD related bearer capabilities

Also adds a test case for both encoder and decoder of this IE
diff --git a/tests/gsm0408/gsm0408_test.ok b/tests/gsm0408/gsm0408_test.ok
new file mode 100644
index 0000000..5ce19e6
--- /dev/null
+++ b/tests/gsm0408/gsm0408_test.ok
@@ -0,0 +1,2 @@
+Test `CSD 9600/V.110/transparent' passed
+Test `Speech, all codecs' passed