[Cashtab] run auto @generated script posttest

Summary: Changed the auto @generated script to run posttest instead of pretest as per review feeback. Related to task T2008

Test Plan:
cd web/cashtab
remove @generated from each snapshot file
npm start
npm test
exit test
ensure @generated is added to each snapshot file

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10577