Respect "all_exec_tests" CLI flag
Currently mjtest seems to always execute big
tests, rendering the CLI flag useless. I happened to write a test that takes a minute to run and would like not to run it by default.
This change seems to have been accidentally introduces in Paul's PR, which unconditially set this variable instead of only doing it for the "all" mode. It seems like your (Johannes) original intention was to run big tests in all
mode, even if no run-all-exec flag was passed.
This PR restores that behaviour (hopefully without introducing another regression).