The Unit suite starts with a freshly-dropped test database and does
not create a $db global, so:
- customer_invoice_email_schema_bootstrap::ensureSchema() short-
circuited and never created the users table.
- runSchemaCheck had nothing to inspect and reported ok=false.
Wire up a real mysqli connection at file load time using the same
CONFIG_DB_* env vars the rest of the CI suite exports, then have
beforeEach run the bootstrap and create the bare-minimum invoices /
bookings tables that runSchemaCheck verifies. This keeps the test
self-contained inside the Unit suite without changing the
production code or the production schema.
Refs: api#383, TRU-77
