feat(server): implement echo core

crates/irtt-server/src/tests/mod.rs

@@ -5,10 +5,15 @@
 //! production encoders wherever the wire form is well-formed, and by hand only
 //! where the point of the test is a payload a compliant encoder cannot produce.
 //!
-//! Tokens come from a scripted source so session identity, collisions and
-//! allocation failure are all assertable.
+//! The two nondeterministic parts are injected. Tokens come from a scripted
+//! source so session identity, collisions and allocation failure are all
+//! assertable; the clock an echo reply's timestamps come from is scripted the
+//! same way, so timestamp behavior is pinned exactly, with no sleeps, no
+//! runtime and no timing tolerances.
 
+mod admission;
 mod close;
+mod echo;
 mod hmac;
 mod no_test;
 mod open;
@@ -16,4 +21,3 @@ mod params;
 mod sessions;
 mod support;
 mod tokens;
-mod unsupported;