For some runtimes, the container is not ready for exec until the
initial container task has been started (as opposed to just having the task created).
More specifically, running containerd-stress with --exec would break
with Kata Container shim, since the sandbox is not created until a
start is issued. By starting the container's primary task before adding
exec's, we can avoid:
```
error="cannot enter container exec-container-1, with err Sandbox not running, impossible to enter the container: unknown"
```
Signed-off-by: Eric Ernst <eric_ernst@apple.com>
8b9571e348