Tests need greater timing window

Hello I ran into this issue building. https://github.com/NixOS/nixpkgs/issues/451983

What we believe is the issue is that the timing related tests have too narrow of a window, and when there is a mass rebuild the system is pegged so much that the tests fail. Tests like this have failed before but we run the tests for a reason. Here's the list of disabled tests.

https://github.com/NixOS/nixpkgs/blob/293d6abedf0478e681a4dfcfcb35b30fc796a32f/pkgs/development/python-modules/sh/default.nix#L41