implement reproducible talloc context printout, i.e. without memory addresses
In some tests (e.g. osmo-msc/tests/msc_vlr/msc_vlr_tests.c) we want to expect a specific non-empty talloc context, and it would be best to print a summary and verify against it in test.ok or test.err output. However, the talloc_report* functions print memory addresses that change for every invocation, and hence cannot be used easily for automatic verification.
Implement a talloc table walker similar to what Vadim has done for the "show talloc-context" VTY code.
Use it in msc_vlr_tests.c to add to expected output, and remove that comment added in https://gerrit.osmocom.org/4927 aka If92a18bb3dc24c2cf6498aa2da29266267488240