msgb/test: Add functions to catch and check exceptions

Currently the msgb error handling cannot be fully tested, since in
many cases osmo_panic will be called. This will in turn call abort().
Using an osmo_panic_handler that just returns will not help, since
many msgb functions rely on MSGB_ABORT to not return at all.

This commit uses an alternative osmo_panic_raise handler that just
calls longjmp to return to the test function.

Since some of this activity is logged to stderr where the strings may
contain variable parts like pointer addresses, stderr checking is
disabled in testsuite.at.

Sponsored-by: On-Waves ehf
diff --git a/tests/testsuite.at b/tests/testsuite.at
index d9bd323..9cda1de 100644
--- a/tests/testsuite.at
+++ b/tests/testsuite.at
@@ -36,7 +36,7 @@
 AT_SETUP([msgb])
 AT_KEYWORDS([msgb])
 cat $abs_srcdir/msgb/msgb_test.ok > expout
-AT_CHECK([$abs_top_builddir/tests/msgb/msgb_test], [0], [expout])
+AT_CHECK([$abs_top_builddir/tests/msgb/msgb_test], [0], [expout], [ignore])
 AT_CLEANUP
 
 if ENABLE_MSGFILE