Only run 4 tool tests in parallel on CircleCI linux
Only run 4 tool tests in parallel on CircleCI linux

Summary:
CircleCI's linux run of ./tool test started erroring recently.
I ssh'd in and could repro. It worked fine for me with `-p 8` and I saw
an ENOMEM at some point. Maybe the containers have fewer resources for
some reason. Anyway, let's run only 4 tests at a time to be extra
careful.

Reviewed By: samwgoldman

Differential Revision: D7436531

fbshipit-source-id: 0b8d4cc4bac0acdc4498a868557109925914541c

AgentME/flow,mroch/flow,AgentME/flow,JonathanUsername/flow,gabelevi/flow,gabelevi/flow,nmote/flow,claudiopro/flow,popham/flow,AgentME/flow,popham/flow,facebook/flow,popham/flow,samwgoldman/flow,nmote/flow,jamesgpearce/flow,jamesgpearce/flow,mroch/flow,mroch/flow,samwgoldman/flow,popham/flow,gabelevi/flow,nmote/flow,nmote/flow,jamesgpearce/flow,JonathanUsername/flow,facebook/flow,JonathanUsername/flow,nmote/flow,claudiopro/flow,claudiopro/flow,facebook/flow,jamesgpearce/flow,jamesgpearce/flow,gabelevi/flow,popham/flow,samwgoldman/flow,claudiopro/flow,samwgoldman/flow,mroch/flow,samwgoldman/flow,JonathanUsername/flow,gabelevi/flow,facebook/flow,mroch/flow,samwgoldman/flow,popham/flow,JonathanUsername/flow,JonathanUsername/flow,nmote/flow,gabelevi/flow,nmote/flow,facebook/flow,jamesgpearce/flow,mroch/flow,samwgoldman/flow,claudiopro/flow,AgentME/flow,mroch/flow,facebook/flow,jamesgpearce/flow,facebook/flow,JonathanUsername/flow,gabelevi/flow,AgentME/flow,popham/flow