[Cashtab]Auto add @generated to top of updated snapshot files

Summary: Bash script which checks snapshot files for str @generated on first line, if not found it is added. Runs prior to npm run test. Related to task T1918.

Test Plan:
Remove @generated from top snapshot files
npm run test || npm run pretest
ensure that 7 files now have @generated appended to first line
npm run test || npm run pretest
ensure that 7 files do not now have a second @generated appended to first line

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10397