Talk about tests and benchmarks

Author: Philippe-Cholet

CONTRIBUTING.md

@@ -10,6 +10,9 @@ then **please first file an issue** describing your proposal._
 To pass CI tests successfully, your code must be free of "compiler warnings" and "clippy warnings" and be "rustfmt" formatted.
 
 ## Write a new method/adaptor for `Itertools` trait
+In general, the code logic should be tested with [quickcheck](https://crates.io/crates/quickcheck) tests in `tests/quick.rs`
+which allow us to test properties about the code with randomly generated inputs.
+
 ### Behind `use_std`/`use_alloc` feature?
 If it needs the "std" (such as using hashes) then it should be behind the `use_std` feature,
 or if it requires heap allocation (such as using vectors) then it should be behind the `use_alloc` feature.
@@ -32,6 +35,8 @@ Iterators are [lazy](https://doc.rust-lang.org/std/iter/index.html#laziness):
 - structs of iterator adaptors should have `#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]` ;
 - structs of iterators should have `#[must_use = "iterators are lazy and do nothing unless consumed"]`.
 
+Those behaviors are **tested** in `tests/laziness.rs`.
+
 ## Specialize `Iterator` methods
 It might be more performant to specialize some methods.
 However, each specialization should be thoroughly tested.
@@ -63,6 +68,9 @@ Additionally,
 
 An adaptor might use the inner iterator specializations for its own specializations.
 
+They are **tested** in `tests/specializations.rs` and **benchmarked** in `benches/specializations.rs`
+(build those benchmarks is slow so you might want to temporarily remove the ones you do not want to measure).
+
 ## Additional implementations
 ### The [`Debug`](https://doc.rust-lang.org/std/fmt/trait.Debug.html) implementation
 All our iterators should implement `Debug`.
@@ -159,6 +167,8 @@ The conditions on which it should be implemented are usually the ones from the `
 ### When to implement [`itertools::PeekingNext`](https://docs.rs/itertools/latest/itertools/trait.PeekingNext.html)
 TODO
 
+This is currently **tested** in `tests/test_std.rs`.
+
 ## About lending iterators
 TODO