[Cashtab] Update snapshots without manual input
[Cashtab] Update snapshots without manual input

Summary:
Related to T2088

Cashtab diffs are frequently pushed with un-updated snapshots causing avoidable teamcity errors.

This diff adds the `-u` flag to the jest `npm test` command, which auto-updates snapshots every time `npm test` is run (before, the user would have to wait for snapshot tests to fail, then push `u`).

Getting this automated will reduce admin burden and is a stepping stone toward including other cashtab linting in `arc lint`.

Test Plan:
1. Edit the file `Send.js` by changing some of the rendered text, e.g. by changing the text "Switch to multiple recipients" to something else
2. Run `npm test`
3. Note that new snapshots are created automatically and tests do not fail.
4. Revert your original changes to `Send.js`
5. Run `npm test`
6. Note that new snapshots are created automatically and tests do not fail (and they are equivalent to the previous snapshots)

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10786

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