fix test for mcmc_recover_scatter()

Running the unit test for mcmc_recover_scatter() on the master branch gives an error.

```
test-mcmc-recover.R:64: warning: mcmc_recover_scatter returns a ggplot object
no non-missing arguments to min; returning Inf

test-mcmc-recover.R:64: warning: mcmc_recover_scatter returns a ggplot object
no non-missing arguments to max; returning -Inf
```