Prevent resetMocks: true from clearing mock implementations

[michaeldfallen]

Requirements

• [x] I have added test coverage for new or changed functionality
• [x] I have followed the repository's pull request submission guidelines
• [ ] I have validated my changes against all supported platform versions

Describe the solution you've provided

If a team uses resetMocks: true in their jest config then the mocked functions of launchdarkly-react-client-sdk will return undefined during test execution.

To allow teams using resetMocks to use this package I have pulled the mock implementations into a setupMocks function that can be called in beforeEach.

I haven't updated the documentation to explain this usecase, I'd value feedback on where / how to do that.

[yusinto]

Thank you for submitting this PR. We will review and provide an update soon.