Add simple tool for TCP loopback testing

Monitor your board's serial log for the IP address it's assigned. Then,
pass it into this command-line tool to make sure you can connect, send a
message, and receive the response.

Cargo.toml

@@ -87,6 +87,7 @@ eh02-unproven = []
 members = [
     "board",
     "logging",
+    "tools",
 ]
 
 [workspace.dependencies]

tools/Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+name = "tools"
+version = "0.1.0"
+repository.workspace = true
+keywords.workspace = true
+categories.workspace = true
+license.workspace = true
+edition.workspace = true
+publish = false
+
+[dependencies]

tools/src/bin/tcp_loopback.rs

@@ -0,0 +1,24 @@
+use std::io::prelude::*;
+use std::net::TcpStream;
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let ip = std::env::args()
+        .skip(1)
+        .next()
+        .ok_or("Provide an IPv4 address")?;
+
+    let ip: std::net::Ipv4Addr = ip.parse()?;
+
+    let mut client = TcpStream::connect(std::net::SocketAddrV4::new(ip, 5000))?;
+
+    const MSG: &[u8] = b"Hello, world!";
+    client.write(MSG)?;
+    let mut resp = [0; MSG.len()];
+    client.read(&mut resp)?;
+
+    if resp == MSG {
+        Ok(())
+    } else {
+        Err(format!("Expected '{MSG:?}' but received '{resp:?}'").into())
+    }
+}