Unify some testing interfaces

This tidies up our testing tools a little bit.  We had several
functions which through various changes ended up doing the same and
some more which did very similar stuff, so I merged them to have
things simpler.  Also moved towards methods on the TestContext struct
while cleaning this up anyway, seems like this structure is going to
stay around for a bit anyway.

The intersting change is in `test_utils.rs`, everything else is just
updating callers.  A few tests used example.org which I moved to
example.com to be able to re-use more configuration of the test
context.

src/message.rs

@@ -1645,7 +1645,7 @@ mod tests {
     async fn test_prepare_message_and_send() {
         use crate::config::Config;
 
-        let d = test::dummy_context().await;
+        let d = test::TestContext::new().await;
         let ctx = &d.ctx;
 
         let contact = Contact::create(ctx, "", "dest@example.com")
@@ -1669,7 +1669,7 @@ mod tests {
 
     #[async_std::test]
     async fn test_get_summarytext_by_raw() {
-        let d = test::dummy_context().await;
+        let d = test::TestContext::new().await;
         let ctx = &d.ctx;
 
         let some_text = Some("bla bla".to_string());