Add exit code propagation to the integration tests

Add exit code propagation to the integration tests

Now, if any of the tests fails, the dlog_test script will return a
nonzero error code. You can use this, for example, to execute
`dlog_test pipe && dlog_test logger`.

Note that, if you are using SDB, running `sdb shell dlog_test pipe` or a
similar command will NOT pass the exit code to the local shell. This is
because SDB always returns 0 as the exit code, no matter what was the
code of the underlying command; the nonzero exit codes are reserved for,
for example, connection failures.

Change-Id: I935ad63170ce7e02d27b496d1cf62f8c755bc3ab