phd: wait for source to resume before asking to migrate again (#874)

The `import_failure` and `export_failure` PHD tests issue a migration
that's expected to fail. After they do, make them wait for the migration
source to report that it's Running (and so is ready to migrate again)
before actually requesting a second migration.

Author: gjcolombo

phd-tests/tests/src/migrate.rs

@@ -181,6 +181,15 @@ mod running_process {
             .state;
         assert_eq!(target_migration_state, MigrationState::Error);
 
+        // Wait for the source to report that it has resumed before requesting
+        // another migration.
+        source
+            .wait_for_state(
+                InstanceState::Running,
+                std::time::Duration::from_secs(5),
+            )
+            .await?;
+
         // try again. this time, it should work!
         target2
             .migrate_from(&source, Uuid::new_v4(), MigrationTimeout::default())
@@ -234,6 +243,15 @@ mod running_process {
             .state;
         assert_eq!(target_migration_state, MigrationState::Error);
 
+        // Wait for the source to report that it has resumed before requesting
+        // another migration.
+        source
+            .wait_for_state(
+                InstanceState::Running,
+                std::time::Duration::from_secs(5),
+            )
+            .await?;
+
         // try again. this time, it should work!
         target2
             .migrate_from(&source, Uuid::new_v4(), MigrationTimeout::default())