The contiguous numbers thing comes from the regulations governing the format of VAT invoices specifically - a VAT invoice must have “a sequential number based on one or more series which uniquely identifies the document.”
Note the “or more” - it’s allowed to use multiple different formats in parallel (e.g. customerId-serialNo) as long as within each sequence the numbers are consecutive. The intention is that if you get a VAT inspection and the inspector spots invoices “XY-00153” and “XY-00155” then you’d better be able to produce “XY-00154” or have a very good explanation why it’s missing…
Changing the test invoices to test001 and test002 would satisfy this requirement.
Sequential numbering isn’t strictly required if you’re not VAT registered but it’s good practice anyway, if only so you don’t have to change your procedures when you hit the threshold.