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)