TL;DR: this allows to show logs from failed runc restore.

Bats scripts are run with `set -e`. This is well known and obvious,
and yet there are a few errors with respect to that, including a few
"gems" by yours truly.

1. bats scripts are run with `set -e`, meaning that `[ $? -eq 0 ]` is
   useless since the execution won't ever reach this line in case of
   non-zero exit code from a preceding command. So, remove all such
   checks, they are useless and misleading.

2. bats scripts are run with `set -e`, meaning that `ret=$?` is useless
   since the execution won't ever reach this line in case of non-zero
   exit code from a preceding command.

In particular, the code that calls runc restore needs to save the exit
code, show the errors in the log, and only when check the exit code and
fail if it's non-zero. It can not use `run` (or `runc` which uses `run`)
because of shell redirection that we need to set up.

The solution, implemented in this patch, is to use code like this:

```bash
ret=0
__runc ... || ret=$?
show_logs
[ $ret -eq 0 ]
```

In case __runc exits with non-zero exit code, `ret=$?` is executed, and
it always succeeds, so we won't fail just yet and have a chance to show
logs before checking the value of $ret.

In case __runc succeeds, `ret=$?` is never executed, so $ret will still
be zero (this is the reason why it needs to be set explicitly).

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>