scripts/parallel-properties.py mis-reports subprocess ABORT as TIMEOUT

[rod-chapman]

CBMC version: 6.2.0
Operating system: macOS

The parallel-properties.py script mis-reports the result of a crashed/aborted cbmc process as a TIMEOUT in its
output.

For example, on an invariant failure, the cbmc process calls abort() and yields a return code of 134 (SIGABORT). Unfortunately, the Python subprocess module re-interprets this and returns -6. This is then reported as TIMEOUT in the enclosing Python code,
which is mis-leading for a user.

It would also help if the script reported a clear error message if ANY subprocess exited with ABORT during an entire run.