Set the Ltac backtrace option in tests.

This is a change fixing Iris output after https://github.com/coq/coq/pull/9142.

I didn't check, but if warnings are not taken into account in the output tests, this PR should be backward compatible.

It does seem that the test script is robust enough to ignore warnings, so this looks good to go.

CI running at https://gitlab.mpi-sws.org/iris/iris/pipelines/14408.

CI looking good. Thanks!

mentioned in commit eb921edd

merged