Memory increasing in empty project

[alexbarnsley]

🐛 Bug Report

There are technically 2 issues (but this report will focus on the memory issue)

1. Memory increases with every test that is performed.
2. There was a regression between v23 and v24 which resulted in slower tests

To Reproduce

Steps to reproduce the behavior:

Create a loop of many tests and run them. I tested using 1000 tests in 100 files (100k tests), but the same can be done in a single file. Most obvious when using the --runInBand flag along with --logHeapUsage

Expected behavior

Memory usage should at the very least drop after each file instead of continuing to grow.

Link to repl or repo (highly encouraged)

v25

Repository
Tests

v24

Repository
Tests

v23

Repository
Tests

To test the above, clone the repo and run yarn test:debug

envinfo

System:
    OS: macOS 10.15.1
    CPU: (8) x64 Intel(R) Core(TM) i7-7820HQ CPU @ 2.90GHz
  Binaries:
    Node: 12.14.1 - /usr/local/bin/node
    Yarn: 1.17.3 - /usr/local/bin/yarn
    npm: 6.11.2 - ~/.npm-packages/bin/npm
  npmPackages:
    jest: Omitted due to testing v23-25

Despite the above OS/CPU specs, the same can be seen from the linked test results from GitHub Actions.

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