Improve randomized testing setup

* Allow iterations to be specified via an env var
* Randomly decide the edit count, with a maximum
  specified via an env var.
* Instead of separate env vars for starting seed + trial, just accept a seed
* Remove some noisy output

cli/src/tests/helpers/random.rs

@@ -1,5 +1,7 @@
-use rand::distributions::Alphanumeric;
-use rand::prelude::{Rng, SeedableRng, StdRng};
+use rand::{
+    distributions::Alphanumeric,
+    prelude::{Rng, SeedableRng, StdRng},
+};
 
 const OPERATORS: &[char] = &[
     '+', '-', '<', '>', '(', ')', '*', '/', '&', '|', '!', ',', '.',