test: Tune down DELTACHAT_SAVE_TMP_DB hint (#6998)

Follow-up for https://github.com/chatmail/core/pull/6992

Since we're printing the hint to stderr now, rather than stdout (as per
link2xt's suggestion), it was too noisy. Also, it was printed once for
every test account rather than once per test.

Now, it integrates nicely with rust's hint to enable a backtrace:

```
  stderr ───

    thread 'chat::chat_tests::test_broadcasts_name_and_avatar' panicked at src/chat/chat_tests.rs:2757:5:
    assertion failed: `(left == right)`

    Diff < left / right > :
    <true
    >false


    note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
    note: If you want to examine the database files, set environment variable DELTACHAT_SAVE_TMP_DB=1

        FAIL [   0.265s] deltachat chat::chat_tests::test_broadcast
```

src/test_utils.rs

@@ -1082,8 +1082,6 @@ impl Drop for TestContext {
                             .join(format!("test-account-{}.db", self.name()));
                         tokio::fs::copy(from, &target).await.unwrap();
                         eprintln!("Copied database from {from:?} to {target:?}\n");
-                    } else {
-                        eprintln!("Hint: If you want to examine the database files, set environment variable DELTACHAT_SAVE_TMP_DB=1\n")
                     }
                 });
             }
@@ -1168,6 +1166,11 @@ impl Drop for InnerLogSink {
         while let Ok(event) = self.events.try_recv() {
             print_logevent(&event);
         }
+        if std::env::var("DELTACHAT_SAVE_TMP_DB").is_err() {
+            eprintln!(
+                "note: If you want to examine the database files, set environment variable DELTACHAT_SAVE_TMP_DB=1"
+            )
+        }
     }
 }