test: stabilize program tests locale

[cyphercodes]

Fixes #3676

Summary

• Force a stable English locale for program.Program unit tests that assert on yargs validation messages.
• Restore the caller's locale environment afterward so other tests and product behavior are unchanged.

Verification

• npm ci
• npm run build
• LANG=de_DE.UTF-8 ./node_modules/.bin/mocha tests/unit/test.setup.js tests/unit/test.program.js --grep 'reports unknown commands|throws an error about unknown commands'

[Rob--W]

Could we keep it simple and only set the LC_ALL environment variable? If set all others are not used. Yargs's relevant source is https://github.com/yargs/yargs/blob/437f3a4e0f4166e1f1a3ce023b0331159582746d/lib/yargs-factory.ts#L1593