rust: translated random test of missingancestors
This is a Rust implementation of the random
DAG generator and related incrementalmissingancestors
tests against a naive brute force implementation.
It is provided as an integration test, so that it
won't run by default if any unit test fails.
In case of a failed example, all needed information
for reproduction is included in the panic message,
(this is how
test_remove_ancestors_from_case1() has been generated),
as well as the random seed.
The whole test is rerunnable by passing the random seed
in the TEST_RANDOM_SEED environment variable.
The other parameters (numbers of iterations) can be passed
in the TEST_MISSING_ANCESTORS environment variable.
An alternative would have been to expose to Python
MissingAncestors<VecGraphs> but that would have meant
pollution of the release build used from Python,
whereas we do it in this changeset within the tests submodule
Differential Revision: https://phab.mercurial-scm.org/D5417