Allow `podman exec` with `--interactive` and no `--tty`

[jhc4318]

An interactive container does not need to have a TTY. There's a slight difference in behaviour that is currently a blocker for my use case.

>podman exec -i ctr_name /bin/bash -c 'hi' 1> 1.txt 2> 2.txt
>cat 1.txt
>cat 2.txt
/bin/bash: hi: command not found

>podman exec -it ctr_name /bin/bash -c 'hi' 1> 1.txt 2> 2.txt
>cat 1.txt 
/bin/bash: hi: command not found
>cat 2.txt 

I've added a testcase for the --interactive only case however stdout/stderr isn't captured by PoW in the -it case so no test has been added there (this also does not affect the existing behaviour there anyway).