[Cashtab] Standardize testing regime

Summary:
As part of code quality assurance, there is a need to develop a minimum test plan for Cashtab that touches on:

• interactions with fresh new wallets
• interactions with wallets which have old TXs relevant to the diff
• regression testing of functions relating to the diff
• cross browser compatibility beyond chrome
• testing on physical mobile devices

We've seen some edge cases recently around these scenarios so will be ideal to have a consistent minimum test plan coverage before the Diff gets to reviewers. This will become more important as we onboard more developers and reviewers get busier.

Manifest: T2041

Test Plan: navigate to the https://github.com/Bitcoin-ABC/bitcoin-abc/tree/master/web/cashtab readme page.

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10864