test: transfer vCards in TestContext.create_chat()

SecureJoin and importing a vCard are the primary
ways we want to support for creating contacts.
Typing in an email address and relying on Autocrypt
results in sending the first message unencrypted
and we want to clearly separate unencrypted and encrypted
chats in the future.

To make the tests more stable, we set up test contacts
with vCards as this always immediately
results in creating a single encrypted chat
and this is not going to change.

src/receive_imf/receive_imf_tests.rs

@@ -1,3 +1,4 @@
+use rand::Rng;
 use std::time::Duration;
 
 use tokio::fs;
@@ -1894,7 +1895,6 @@ async fn test_save_mime_headers_off() -> anyhow::Result<()> {
 
     let msg = bob.recv_msg(&alice.pop_sent_msg().await).await;
     assert_eq!(msg.get_text(), "hi!");
-    assert!(!msg.get_showpadlock());
     let mime = message::get_mime_headers(&bob, msg.id).await?;
     assert!(mime.is_empty());
     Ok(())
@@ -4642,7 +4642,14 @@ async fn test_download_later() -> Result<()> {
 
     let bob = tcm.bob().await;
     let bob_chat = bob.create_chat(&alice).await;
-    let text = String::from_utf8(vec![b'a'; MIN_DOWNLOAD_LIMIT as usize])?;
+
+    // Generate a random string so OpenPGP does not compress it.
+    let text: String = rand::thread_rng()
+        .sample_iter(&rand::distributions::Alphanumeric)
+        .take(MIN_DOWNLOAD_LIMIT as usize)
+        .map(char::from)
+        .collect();
+
     let sent_msg = bob.send_text(bob_chat.id, &text).await;
     let msg = alice.recv_msg(&sent_msg).await;
     assert_eq!(msg.download_state, DownloadState::Available);