tests: Fix signal-desktop

[devhell]

This test fails due to OOM on the VM. Setting the memory of the VM to
1024 lets the test succeed.

Cc: @flokli

Motivation for this change

The test failed repeatedly on a private Hydra instance and when tested on a local machine.

Things done

Set virtualisation.memorySize = 1024;

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[devhell]

@flokli Sorry, I don't mean to be pushy, but it would be great if this test could be fixed. ;-)

[flokli]

Sorry, I fully agree. This also probably should get into 20.03 and 19.09, shouldn't it?

[flokli]

backported to 20.03 in 2ba4461, to 19.09 in 84af403.