added instrumented test for Prompt with docs

[peterjangml]

I've added instrumented test for Prompt (with Javadoc). TimerPromptInstrumentedTest requires review with @yoyo-gong before final submission, so it has not been added in current PR (projected to be added for next PR).

Added: RegistrationPromptInstrumentedTest and SigninPromptInstrumentedTest

• Added test cases for various cases, such as success, invalid credentials, and missing fields
• These test cases can be improved if removeUser() is implemented in the future
• Developed mainly using Espresso and JUnit

[yoyo-gong]

looks good