docs(scm-bisect): add doc-comments to scm_bisect::testing

`cargo clippy` was failing in this CI run: https://github.com/arxanas/git-branchless/actions/runs/12127041277?pr=1460

Actually, I have no idea how it even resulted in this. If I understand correctly, `cfg(test)` shouldn't even be enabled here, so the `testing` module shouldn't even be checked. I was unable to reproduce the issue locally on `1.74.1` and `nightly`. Possibly it's something non-deterministic that depends on the order that crates were compiled, or some other caching issue.

Regardless, it seems fine to add the doc-comments, so I've added them and now hope that CI will pass.

scm-bisect/src/testing.rs

@@ -1,3 +1,5 @@
+//! Testing utilities.
+
 use std::collections::{HashMap, HashSet};
 use std::convert::Infallible;
 
@@ -7,6 +9,7 @@ use proptest::prelude::*;
 
 use crate::basic::{BasicSourceControlGraph, BasicStrategyKind};
 
+/// Graph that represents a "stick" of nodes from 0 to `max` (exclusive).
 #[derive(Clone, Debug)]
 pub struct UsizeGraph {
     pub max: usize,
@@ -27,8 +30,10 @@ impl BasicSourceControlGraph for UsizeGraph {
     }
 }
 
+/// Directed acyclic graph with nodes `char` and edges `char -> char`.
 #[derive(Clone, Debug)]
 pub struct TestGraph {
+    /// Mapping from parent to children.
     pub nodes: HashMap<char, HashSet<char>>,
 }
 
@@ -57,6 +62,7 @@ impl BasicSourceControlGraph for TestGraph {
     }
 }
 
+/// Select an arbitrary [`BasicStrategyKind`].
 pub fn arb_strategy() -> impl ProptestStrategy<Value = BasicStrategyKind> {
     prop_oneof![
         Just(BasicStrategyKind::Linear),
@@ -65,6 +71,7 @@ pub fn arb_strategy() -> impl ProptestStrategy<Value = BasicStrategyKind> {
     ]
 }
 
+/// Create an arbitrary [`TestGraph`] and an arbitrary set of failing nodes.
 pub fn arb_test_graph_and_nodes() -> impl ProptestStrategy<Value = (TestGraph, Vec<char>)> {
     let nodes = prop::collection::hash_set(
         prop::sample::select(vec!['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']),