Allow job artifacts to be retrieved from the mock

This is useful when testing whether the artifacts uploaded are as
expected.  In particular, we need this now to test whether masked
artifacts are actually being masked as we expect them to be.

Note that we are returning the raw binary of the zipfile, and we'll
need to use the zipfile crate to actually interpret this data. That
seems acceptable for now.

gitlab-runner-mock/src/lib.rs

@@ -134,6 +134,16 @@ impl GitlabRunnerMock {
         jobs.jobs.push(job);
     }
 
+    pub fn get_job_artifact(&self, id: u64) -> Option<Vec<u8>> {
+        let jobs = self.inner.jobs.lock().unwrap();
+
+        jobs
+            .jobs
+            .iter()
+            .find(|j| j.id() == id)
+            .map(|j| j.artifact().as_slice().to_vec())
+    }
+
     fn grab_pending_job(&self) -> Option<MockJob> {
         let jobs = self.inner.jobs.lock().unwrap();
         for job in jobs.jobs.iter() {