Add MM Auth test; add auth_action_str() function

Add basic MM Authentication test setup, with fake DB access and RAND_bytes().

So far implement simple tests for IO error during DB access and missing auth
entry.

To print the auth action during tests, add struct auth_action_names and
auth_action_str() inline function in auth.[hc].
diff --git a/openbsc/.gitignore b/openbsc/.gitignore
index 55f4a31..28fdcc8 100644
--- a/openbsc/.gitignore
+++ b/openbsc/.gitignore
@@ -80,6 +80,7 @@
 tests/subscr/subscr_test
 tests/oap/oap_test
 tests/gtphub/gtphub_test
+tests/mm_auth/mm_auth_test
 
 tests/atconfig
 tests/atlocal