RNTester: inline BorderExample testIDs for simpler maintenance (#41987)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/41987

Previously, the examples allow overriding the testIDs for each inner example from BorderExample.js in RNTester. However, that setup relies on another infra/abstraction to inject the testIDs properly. For simplicity, let's just make them hardcoded using the pattern `border-test-<example-name>`.

Changelog: [Internal]

Reviewed By: NickGerleman, mdvacca

Differential Revision: D52282922

fbshipit-source-id: 8fdc3d799befddbbb9bd8e60c8a904670c035d59