[test] add mathl config steps test, fix most t/steps tests

we have a new capture API.

also add clean to auto::infnan
[test] disable some recursive t/steps/auto tests

capture is not re-entrant, so we get empty stdout for already cleaned
stdout capture.
Disable these 3 tests for now.
Release 6.11.0 - Fischer's Lovebird

See ChangeLog