tests/init.sh: print exit code of failed commands
tests/init.sh: print exit code of failed commands

* init.sh (run_prog, run_prog_skip_if_failed): When the program fails,
add its exit code to the diagnostic message.
(run_strace): When strace fails, add its exit code to the diagnostic
message.
(run_strace_merge): When strace-log-merge fails, add its exit code
to the diagnostic message.

cuviper/strace,cuviper/strace,Saruta/strace,cuviper/strace,cuviper/strace,Saruta/strace,Saruta/strace,Saruta/strace,Saruta/strace,cuviper/strace