Fix example to contain expectation
Fixes #5449
allow guests to use the payments api for their orders
Fixes #5450
Improve localization of stock_locations/_form
Fixes #5454
Fix factories to generate unique SKUs

* Choosing random SKUs even from a high range can yield duplicates
  resulting in build failures.
* Kernel.rand(9999) was definitively never a good choice due the
  birthday paradoxon lets manifest a duplicate quite often
* Uses sequencially generated SKUs defined from a single shared and
  reusable generator.

Fixes #5452