Allow building without libgtest-dev under <nocheck> build profile

I'd like to avoid pulling libgtest-dev into the bootstrap set.

Fortunately, libgtest-dev is only used for testing apt and apt
correctly implements DEB_BUILD_OPTIONS=nocheck now. So this
bug is about getting rid of the Build-Depends.

Simply removing it (by adding a build profile) is not sufficient
however as configure fails hard, so an additional bit is necessary
to cover for that.

Closes: #809726