sw:vec: test_runner: grab elf return code

Grab the return code from the log string. Ideally we would like to grab
the result from CSR or a register, but QEMU doesn't have the register
access hooked up. :(

With this change, test_runner.py will tunnel the main return code, and
can be used in CI or ctest.

Change-Id: I6dfbf9cb1b4f4911bcfb230936c67f255178ee7c