gnome3 test: increase timeout

The gnome3 test has been failing recently ([1](http://hydra.nixos.org/build/26608126/nixlog/1/raw) [2](http://hydra.nixos.org/build/26605926/nixlog/1/raw)); this is due to exit code 124 which is [the command timing out](https://www.gnu.org/software/coreutils/manual/html_node/timeout-invocation.html).

This increases the timeout to 900, to align with the similar timeout in https://github.com/NixOS/nixpkgs/blob/master/nixos/lib/test-driver/Machine.pm#L222