Add more tests for Realtime Database
### Summary
Increases test coverage, which should be helpful when starting work on v6.
### Checklist
- [x] `e2e` tests added or updated in [/tests/e2e/\*](/tests/e2e)
### Release Plan
[TESTING][ENHANCEMENT][Database] - More tests
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- [ANDROID] [BUGFIX] [DATABASE] - Database listeners now correctly tearing down between RN reloads. (Fixes#1498#1611#1609)
- [JS] [BUGFIX] [DATABASE] - Fixed an issue where `Reference.toString()` incorrectly contains `//` instead of `/` when joining the parent and child paths.
- [JS] [BUGFIX] [DATABASE] - Rework `.push()` behaviour to match WebSDK and correctly return a Reference instance in all scenarios. (Fixes#893#1464#1572)
- [JS] [ENHANCEMENT] [UTILS] - Added a `firebase.utils().database.cleanup()` utility method which removes all database listeners.
