persist: add multi_thread to test_shard_source_*_initial_as_of tests

Looks like with the Tasked commit, these end up in some sort of hang if
there's only one tokio thread.

Author: danhhz

src/persist-client/src/operators/shard_source.rs

@@ -638,7 +638,7 @@ mod tests {
     ///
     /// NOTE: This test is weird: if everything is good it will pass. If we
     /// break the assumption that we test this will time out and we will notice.
-    #[mz_ore::test(tokio::test)]
+    #[mz_ore::test(tokio::test(flavor = "multi_thread"))]
     #[cfg_attr(miri, ignore)] // unsupported operation: returning ready events from epoll_wait is not yet implemented
     async fn test_shard_source_implicit_initial_as_of() {
         let (persist_clients, location) = new_test_client_cache_and_location();
@@ -707,7 +707,7 @@ mod tests {
     ///
     /// NOTE: This test is weird: if everything is good it will pass. If we
     /// break the assumption that we test this will time out and we will notice.
-    #[mz_ore::test(tokio::test)]
+    #[mz_ore::test(tokio::test(flavor = "multi_thread"))]
     #[cfg_attr(miri, ignore)] // unsupported operation: returning ready events from epoll_wait is not yet implemented
     async fn test_shard_source_explicit_initial_as_of() {
         let (persist_clients, location) = new_test_client_cache_and_location();