Fix finding downstream pipeline by title

Related to #361338 (closed), #361346 (closed), #361339 (closed)

We were looking for downstream pipelines using title attribute but it was removed here. The main purpose of this MR is to address that. I also try to have all the jobs in 1 stage so that they can all start at the same time and hopefully yield less test runtime.

And since I made some changes around api_variable_inheritance_with_forward_pipeline_variables_spec.rb, I also lift its quarantine here as well. I want to put it into the transient bucket for better monitoring with the new wait logics. This is related to #361400 (closed)

app/assets/javascripts/pipelines/components/graph/linked_pipeline.vue

@@ -237,7 +237,11 @@ export default {
         <div
           class="gl-display-flex gl-downstream-pipeline-job-width gl-flex-direction-column gl-line-height-normal"
         >
-          <span class="gl-text-truncate" data-testid="downstream-title">
+          <span
+            class="gl-text-truncate"
+            data-testid="downstream-title"
+            data-qa-selector="downstream_title_content"
+          >
             {{ downstreamTitle }}
           </span>
           <div class="gl-text-truncate">