test: kill plymouthd after initrd transition if it's still running

test: kill plymouthd after initrd transition if it's still running

Until now using the INTERACTIVE_DEBUG=yes stuff together with sanitizers
was almost impossible, since the console kept eating up our inputs or
not responding at all. After a painful day of debugging I noticed that
if we use a shell script in the initrd -> root transition, we might end up
with a plymouthd still running, which kept screwing with the tty.

E.g. with initrd -> wrapper -> systemd transition, where the `wrapper`
is a simple script:

```
exec -- /usr/lib/systemd/systemd "$@"
```

we'd end up with a stray plymouthd process after the bootup:

```
 1     0     440       2  20   0      0     0 worker I    ?          0:00 [kworker/5:2-ata_sff]
 1     0     453       2  20   0      0     0 worker I    ?          0:00 [kworker/9:2-rcu_gp]
 5     0     456       1  20   0   7252  1960 do_epo S    ?          0:00 @usr/sbin/plymouthd --mode=boot --pid-file=/run/plymouth/pid --attach-to-session
```

After killing it, the tty works finally as expected.