Instead of having to manually stub packages that should not be
downloaded we instead automatically stub all packages (except a small
list of whitelisted ones). Tests can re-introduce the real package by
using the `realPkgs` module argument.
M_SHARE is not a valid column on Darwin. It seems that previously htop
ignored unknown columns, but the current version does not display all
subsequent columns.
This option provides a more convenient way to overlay dummy packages.
It also adds a function `config.lib.test.mkStubPackage` that can,
e.g., be used for `package` options.
When the 'fields' setting is not set in htoprc, the htop program won't read any
of the settings. Provide a default value for fields in case it's not explicitly
set by the user.
