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.
Before the XDG variables would be set from the user's environment, if
available. This would break some tests.
With this change the tests should be fully deterministic.
Fixes#1190
This resolves the error
The option `accounts.email.accounts.xyz.neomutt.sendMailCommand`
is defined both null and not null, in
`…/home-manager/modules/accounts/email.nix' and
`…/home-manager/modules/accounts/email.nix'.
that would occur previously when both neomutt and msmtp were enabled
for an account.
