[lldb/Utils] Serialize exit code in lldb-repro.py

After 61d5b0e6 more shell test are expected to exit with a non-zero
status code. Because the exit status is computed in the driver and not
behind the SB API layer, reproducers don't know about it and always
return 0 unless replay failed.

This discrepancy means that these tests don't work with lldb-repro.py
and skipping them for this reason would be a pity. To solve this
problem, the script now serializes the exit code during capture and
returns that during replay.

These is an assert that ensures that replay exits with a zero exit
status to prevent replay failures from being silently ignored.