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/job.rs

@@ -1209,7 +1209,7 @@ mod tests {
         // We want to ensure that loading jobs skips over jobs which
         // fails to load from the database instead of failing to load
         // all jobs.
-        let t = dummy_context().await;
+        let t = TestContext::new().await;
         insert_job(&t.ctx, -1).await; // This can not be loaded into Job struct.
         let jobs = load_next(
             &t.ctx,
@@ -1231,7 +1231,7 @@ mod tests {
 
     #[async_std::test]
     async fn test_load_next_job_one() {
-        let t = dummy_context().await;
+        let t = TestContext::new().await;
 
         insert_job(&t.ctx, 1).await;