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/keyring.rs

@@ -79,8 +79,8 @@ mod tests {
     #[async_std::test]
     async fn test_keyring_load_self() {
         // new_self() implies load_self()
-        let t = dummy_context().await;
-        configure_alice_keypair(&t.ctx).await;
+        let t = TestContext::new().await;
+        t.configure_alice().await;
         let alice = alice_keypair();
 
         let pub_ring: Keyring<SignedPublicKey> = Keyring::new_self(&t.ctx).await.unwrap();