Merge branch 'julien/bugfix/run_execnow_several_times' into 'master'

[tests] execnow specified in test_config is now run on each test case

Patch related to MR frama-c/frama-c!728.
@kvorobyov : fix the issue that you talked me about. The main patch is in Frama-C's ```ptests```.

See merge request !35