Generate test results in a discoverable location (#27435)

Summary:
Test results were being written to ~/reports/, while Circle CI was configured to look for these in ~/react-native/reports/.

This simple fix makes it so that Circle CI's test result tab can now display line-by-line results without the need to dig through log spew to understand which tests failed.

## Changelog

[Internal] [Fixed] - Generate test results in discoverable location (CI)
Pull Request resolved: https://github.com/facebook/react-native/pull/27435

Test Plan:
See https://app.circleci.com/jobs/github/hramos/react-native/7112/tests
Compare with any other run, where this tab would show an error message due to lack of test results.

Differential Revision: D19185408

Pulled By: hramos

fbshipit-source-id: 5481a13874bd078deba4a58ddb83dfdfe6845151