Add e2e tests for React Native 0.64 on Android

[twometresteve]

Goal

Adds e2e tests for React Native 0.64 on Android. iOS tests are currently failing, pending a follow-on ticket to fix them.

Design

The same approach has been taken as for RN 0.63 and this is now documented in the TESTING.md.

Changeset

• New test fixture for RN 0.64 (the first commit contains the files generated by a fresh react-native init and the changes from there onwards are the manual changes made).
• New pipeline steps to build and run tests for both Android and iOS (but soft failing on iOS for now)
• Instructions written on how to generate new test fixtures.

Testing

Covered by CI.

[github-actions]

@bugsnag/browser bundle size diff

	Minified	Minfied + Gzipped
Before	41.14 kB	12.66 kB
After	41.14 kB	12.66 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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