[apps] Consistently apply naming convention for mocha unit test files

Summary: Using '.test.js' is picked up by VScode's GUI and imo is easier to parse as "this file is the test file for the other file with the same name without the '.test'" -- esp vs the camelCase _Tests everywhere

Test Plan:
Verify only change is file names
npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14511