Bump test timeout to attempt fixing plot flakes

[lionel-]

Attempt at fixing the plot failures we see on the Windows CI.

I took a look at some of the failing tests and could not see any async issues. So possibly the plots are taking a very long time? We have a 10s timeout, which seems like it should be amply sufficient. But in:

https://github.com/posit-dev/ark/actions/runs/21667681084/job/62467251434#step:9:701

I see successful tests with long running times, e.g. the ragg test here:

        PASS [   1.953s] (638/757) ark::plots test_graphics_device_initialization
        PASS [   4.280s] (639/757) ark::plots test_inability_to_load_ragg_falls_back_to_base_graphics
        FAIL [  11.777s] (640/757) ark::plots test_basic_plot

The PR bumps the timeout to 30s. Which will unfortunately slow down CI reports when real timeouts are hit due to bugs during development. Should hopefully fix the intermittent failures though.