Test suite: don't rely on the state of the APT lists to check if a package is installed (refs: #14596)

I've seen cases where "apt-cache policy" would fail, when run by
'the package "cowsay" is not installed', as part of
"I am notified when Additional Software fails to install a package",
with:

  E: Malformed Description-md5 line; doesn't have the required length (32 != 29)
  '0143a1c3acbdb045e4fcaab0d8657"

This seems wrong and I don't know why this happens, but let's not get
side-tracked: the goal here is to check whether a given package is installed,
not to validate the state of the APT lists. So let's use dpkg instead.

For details about why "dpkg -s" was initially replaced with "apt-cache policy",
see c77c2dd9. The analysis of the problem in
that commit message was correct: "deinstall" means "The package is selected for
deinstallation" and is indeed a transient state. The solution did work until we
face a situation when the higher level tool fails while checking things we're
not interested in.