bashCompletion: speed-up test execution by using xdist
bashCompletion: speed-up test execution by using xdist

Currently the tests take an eternity and are also sometimes flaky. By
following upstream in using xdist for parallel test execution we at
least get the feedback cycle down. On my machine that means instead of
running this for ~25min it runs in 1 minute and 10 seconds.

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