integration-tests: assert cargo-insta output

Clarify that `cargo insta test` is expected to produce a change in the
snaphots.

cargo-insta/integration-tests/src/main.rs

@@ -1,7 +1,7 @@
 use std::env;
 use std::fs;
 use std::path::Path;
-use std::process::Command;
+use std::process::{Output, Command};
 
 use dircpy::CopyBuilder;
 use insta::{assert_snapshot, Settings};
@@ -30,13 +30,23 @@ fn main() {
         .unwrap();
 
     // run tests and accept snapshots
-    let status = Command::new("../target/debug/cargo-insta")
+    let Output {
+        status,
+        stdout,
+        stderr,
+    } = Command::new("../target/debug/cargo-insta")
         .arg("test")
         .arg("--accept")
         .arg("--no-ignore")
-        .status()
+        .output()
         .unwrap();
-    assert!(status.success());
+    let stdout = String::from_utf8(stdout);
+    let stderr = String::from_utf8(stderr);
+    assert!(status.success(), "stdout={:?}\nstderr={:?}", stdout, stderr);
+    let stdout = stdout.unwrap();
+    let stderr = stderr.unwrap();
+    assert!(stdout.contains("accepted:\n"), "{}", stdout);
+    assert!(stderr.contains("Compiling integration-tests"), "{}", stderr);
 
     // use insta itself to assert snapshots
     for entry in WalkDir::new("test-input") {