nixosTests.airsonic: fix failure (type error)
nixosTests.airsonic: fix failure (type error)

airsonic_is_up should return a bool, but machine.succeed returns a
string causing testScriptWithTypes to fail. This is fixed by executing
the cmd with machine.execute and checking the status code.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs