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

Author: gabelevi

.circleci/config.yml

@@ -200,7 +200,7 @@ jobs:
           command: (cd packages/flow-dev-tools && yarn install | cat) # Force no tty mode
       - run:
           name: Run tool tests
-          command: ./tool test --bin bin/linux/flow | cat
+          command: ./tool test -p 4 --bin bin/linux/flow | cat # Limit parallelism
 
   tool_test_macos:
     macos: