[Cashtab] Update SendToken tests to new standardized wrapper

Summary:
T3444

Implement new standardized test wrapper in SendToken tests. Stop using data-testid in SendToken tests and delete from SendToken.

Update placeholder text for Burn to improve both the test and the UI.

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15559