The conversion from `concatMapStrings` to `concatStringsSep` introduced in https://github.com/nix-community/home-manager/pull/2481
creates an unintended behavior change where the formatted config does not end in a newline.[1]
This is problematic for manipulation at the Nix level. In particular, this cause a regression in
the generation of gtk2 settings due to concatenated of the formatted config and `gtk2.extraConfig`
without a newline in between.
This commit restores `concatMapStrings` to match the previous behavior and adds a newline to
the final string for the generated gtk2 config. The test case for gtk2-basic-config
was also updated to check behavior at concatenation boundaries.
[1] - https://github.com/nix-community/home-manager/pull/2481#discussion_r830648706
46dc2e5d9f