Method _partitionTests

packages/jest-core/src/TestScheduler.ts:356–380

(
    testRunners: Record<string, JestTestRunner>,
    tests: Array<Test>,
  )

  }

  private _partitionTests(
    testRunners: Record<string, JestTestRunner>,
    tests: Array<Test>,
  ): Record<string, Array<Test>> | null {
    if (Object.keys(testRunners).length > 1) {
      return tests.reduce((testRuns, test) => {
        const {config} = test.context;
        const runnerKey = `${config.runner}\0${stableRunnerOptionsKey(config.runnerOptions)}`;
        if (!testRuns[runnerKey]) {
          testRuns[runnerKey] = [];
        }
        testRuns[runnerKey].push(test);
        return testRuns;
      }, Object.create(null));
    } else if (tests.length > 0 && tests[0] != null) {
      // If there is only one runner, don't partition the tests.
      const {config} = tests[0].context;
      const runnerKey = `${config.runner}\0${stableRunnerOptionsKey(config.runnerOptions)}`;
      return Object.assign(Object.create(null), {
        [runnerKey]: tests,
      });
    } else {
      return null;
    }
  }

  async _setupReporters(tests: Array<Test>) {
    const {collectCoverage: coverage, notify} = this._globalConfig;