flaky jest --detectLeaks

[Akuukis]

🐛 Bug Report

Further investigation on #8247 reveals that jest --detectLeaks is flaky. Across 14 out of our ~40 testsuites are known to be memory leaking according to #8247. So I skipped those with describe.skip at top level.

Nevertheless, despite not running any tests, repeated jest --detectLeaks reported between 9 and 11 to be memory leaking.

``jest --detectLeaks --watch` has interesting behavior. When watching one of the flaky testsuites, it consistently iterated between "skipped" and "failed". After deleting something it went to randomly failing around 10% of time.

To Reproduce

See #8247.

Expected behavior

Consistent behavior.

Link to repl or repo (highly encouraged)

Sorry, can't post all repo online. Tried to make little contained case but didn't succeed. It was not exclusive to using Sequelize as in #8247.

Run npx envinfo --preset jest

Paste the results here:

  System:
    OS: Linux 4.15 Ubuntu 18.04.2 LTS (Bionic Beaver)
    CPU: (16) x64 AMD Ryzen 7 2700X Eight-Core Processor
  Binaries:
    Node: 8.15.0 - /tmp/yarn--1553787598857-0.15377962644583643/node
    Yarn: 1.15.2 - /tmp/yarn--1553787598857-0.15377962644583643/yarn
    npm: 6.4.1 - /usr/local/bin/npm

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