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

src/plugins/e-acsl/tests/e-acsl-runtime/test_config

 LOG: gen_@PTEST_NAME@.c
 OPT: -machdep gcc_x86_64 -check -e-acsl -then-last -load-script tests/print.cmxs -print -ocode tests/e-acsl-runtime/result/gen_@PTEST_NAME@.c -kernel-verbose 0 -val -value-verbose 0
-EXECNOW: ./scripts/testrun.sh @PTEST_NAME@ e-acsl-runtime "" "--frama-c=@frama-c@"
+EXEC: ./scripts/testrun.sh @PTEST_NAME@ e-acsl-runtime "" "--frama-c=@frama-c@"