Add getByPlaceholder query functionality

[danahartweg]

Summary

There is a helper that exists in react-testing-library to query inputs by their placeholder. This PR adds that support.

You could achieve this by querying for a placeholder prop, or querying for react native TextInputs manually and then checking their props, but it felt cleaner to have this contained in the testing library itself. It will also allow you to bypass any other instances that may have a placeholder prop that aren't TextInputs.

If we don't want to expand the surface of the library further, I completely understand that and am fine with this being closed. Just seems like it may be useful and save some time for several scenarios =)

Test plan

• Render a TextInput with a placeholder prop
• Query the test instance via getByPlaceholder for the input
• Verify the test instance is found

[thymikee]

I think it makes sense, especially that RTL has it already. Could you also add a TS test?

[thymikee]

Mind adding a test here as well? https://github.com/callstack/react-native-testing-library/blob/master/typings/__tests__/index.test.tsx

[thymikee]

Thank you so much for this contribution!

[danahartweg]

You're very welcome! As I use the library more, I'll be sure to keep a list of things that could be improved / added, and then get to discussing or implementing them.