Failing samplers during tests

[sbfnk]

Tests show sampling failures such as

[1] "Error in sampler$call_sampler(args_list[[i]]) : "
[2] "  reached elapsed time limit"                    
[1] "error occurred during calling the sampler; sampling not done"
[1] "Error in sampler$call_sampler(args_list[[i]]) : "
[2] "  reached elapsed time limit"                    
[1] "error occurred during calling the sampler; sampling not done"
[1] "Error in sampler$call_sampler(args_list[[i]]) : "
Execution halted
[2] "  reached elapsed time limit"                    
[1] "error occurred during calling the sampler; sampling not done"
[1] "Error in sampler$call_sampler(args_list[[i]]) : "
[2] "  reached elapsed time limit"                    
[1] "error occurred during calling the sampler; sampling not done"

or

Warning ('test-estimate_secondary.R:24:1'): (code run outside of `test_that()`)
There were 3402 divergent transitions after warmup. See
https://mc-stan.org/misc/warnings.html#divergent-transitions-after-warmup
to find out why this is a problem and how to eliminate them.
Backtrace:
    ▆
 1. └─EpiNow2::estimate_secondary(...) at test-estimate_secondary.R:24:0
 2.   ├─rstan::sampling(...)
 3.   └─rstan::sampling(...)
 4.     └─rstan (local) .local(object, ...)
 5.       └─rstan:::throw_sampler_warnings(nfit)

They occur on all platforms so don't seem to be behind the specific issues on windows (#420) or Ubuntu (#428) but it would probably still be good to avoid theses.