Add RN CLI tests for RN 0.64 (non-Hermes)

[twometresteve]

Goal

Adds tests to the RN CLI CI pipeline for RN 0.64 (non-Hermes).

Design

Follows the existing approach for previous version of React Native.

Changeset

• New test fixture for RN 0.64.
• Required adjustments to inited test fixture app now detailed in TESTING.md
• Build steps added to pipeline

The overall changeset is probably quite hard to review due to all the new test fixture files. Individual commits should be fairly straightforward to scan through.

Testing

Covered by CI.

[github-actions]

@bugsnag/browser bundle size diff

	Minified	Minfied + Gzipped
Before	41.18 kB	12.67 kB
After	41.18 kB	12.67 kB
±	No change	No change

code coverage diff

Coverage values did not change👌.

Total:

Lines	Branches	Functions	Statements
0%(+0%)	0%(+0%)	0%(+0%)	0%(+0%)

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