Nixpkgs: nixos/tests/keymap: non-deterministic failures

[xeji]

Issue description

The keymap tests fail non-deterministically on hydra, which frequently delays nixos channels.
Applies to master and 18.03. These failures have occured since at least December 2017.
Some recent examples:

• master: https://hydra.nixos.org/build/72687103
• 18.03: https://hydra.nixos.org/build/73077967, https://hydra.nixos.org/build/73075854

I believe this is caused by fragile test cases rather than a real problem with the keymaps. Will investigate and try to improve the tests.

[Mic92]

Maybe they could be simplified somehow.

[srhb]

@xeji Cool! You might find some of the work and discussion here useful:

https://github.com/NixOS/nixpkgs/pull/33719

[srhb]

39400 was reverted in release-18.03 due to another non-deterministic failure.

39473 attempts to alleviate this, and we're tracking its progress on Hydra for backport into stable.

[xeji]

While we wait for Hydra I'll prepare a separate PR picking both changes to 18.03 and test it locally.

[srhb]

Let's (arbitrarily) consider 73342244 and 73334828 the indicators on whether to proceed to 18.03, once they're done. Looks good so far.

https://hydra.nixos.org/job/nixos/trunk-combined/tested#tabs-constituents

[srhb]

Aside from a cancellation that I don't think we should count, this looks really good. Let's go for 18.03. :)