Reduce runtime of sim_gs_n() examples

[jdblischak]

From R CMD check on the latest commit 99f1aed running on Ubuntu with R-devel:

* checking examples ... [34s/36s] OK
Examples with CPU (user + system) or elapsed time > 5s
           user system elapsed
sim_gs_n 15.684  0.388  17.764
* checking examples with --run-donttest ... [37s/41s] OK

To reduce the runtime, I used wrapped most of the tests in \donttest{} (which is what we previously did for sim_fixed_n() in #177).

[jdblischak]

Note I had also tried reducing n_sim from 3 to 1 so that more code would be run during the package check, but this wasn't sufficient to reduce the runtime of the examples.

[jdblischak]

I checked the R CMD check results in all 5 CI jobs. None of them flagged the sim_gs_n() examples as long-running.

[LittleBeannie]

Thanks, @jdblischak !