Investigate flaky test-fs-readfile-tostring-fail on macOS

[mscdex]

• Version: master
• Platform: macOS
• Subsystem: test

This test just failed recently on an osx1010 node. It appears it just timed out?:

not ok 405 parallel/test-fs-readfile-tostring-fail
  ---
  duration_ms: 60.203
  severity: fail
  stack: |-
    timeout

/cc @nodejs/testing

[Trott]

On an idle machine, this test is fast. But it gets slow if I run multiple instances of it in parallel. I wonder if it should be moved to sequential so that it does not compete for resources with other tests.

Vague benchmark: If I run 96 instances of this test simultaneously, it takes about 55 seconds to complete. If I run 96 instances of test-fs-readfile-empty.js simultaneously, it takes 2 seconds. test-fs-readfile-error.js: 7 seconds. test-fs-readfile-fd.js: 2 seconds.

[Trott]

Test fails reliably in CI if 32 instances are run at once. (Might not even need that many.) https://ci.nodejs.org/job/node-stress-single-test/1101/nodes=osx1010/console

Same job moved to sequential is reliable: https://ci.nodejs.org/job/node-stress-single-test/1102/nodes=osx1010/console

Will submit a PR to move it.