feat(test-lib): add notFileContains helper

[AlexLov]

This adds a complementary helper (to already existed fileContains) -- fileNotContains for simple comparison.

Generally it just negates result of grep expression from fileContains helper.

[MacaylaMarvelous81]

Imperative test

[jomarm@dell-inspiron7773:~/nix-wrapper-modules]$ nix-instantiate --eval --expr '(((import <nixpkgs> {}).callPackage ./ci/test-lib.nix { self = import ./. {}; }).fileNotContains "maintainers/default.nix" "alexlov").cond'
"! grep -Eq -- 'alexlov' \"maintainers/default.nix\""

[jomarm@dell-inspiron7773:~/nix-wrapper-modules]$ ! grep -Eq -- 'alexlov' "maintainers/default.nix"; echo $?
1

[zenoli]

Thanks for the update. LGTM!

@BirdeeHub I think this is ready to be merged.