tests: fix the spontaneous dpkg frontend lock failures

[MartinLoeper]

E: Could not get lock /var/lib/dpkg/lock-frontend - open (11: Resource temporarily unavailable)
E: Unable to acquire the dpkg frontend lock (/var/lib/dpkg/lock-frontend), is another process using it?

[SinBirb]

while [[ $(lsof /var/lib/dpkg/lock-frontend |& grep unattend) ]]
do
  sleep 2
done
killall -9 apt apt-get

https://itsfoss.com/could-not-get-lock-error/

[MartinLoeper]

Thanks @SinBirb!
I do not know if this works in this case though.
I wait until frontend lock is released before running the installer.
Somehow a background job or so triggers apt-get and makes the intermediate apt calls in the installer script fail.
Any idea what might be running there?