2 containers one on top of the other, the top container is used as an office that has stairs leading to it that is the only way of getting in and out. Do these stairs have to be periodically tested every 6 months like emergency stairs as they are for all purpose use?

The maintenance regime comes from the Facilities Management British Standard BS 8210:2012 for building fabric and equipment and is not specific to a particular elementso would be equally applicable to any stair.

It says:
"Frequency of inspection
Inspections should be carried out as follows:
a) routine: consultation with occupants and other users of the facility to determine the existence of any maintenance matters that might require
action and, where such work has been undertaken, measurement of users’ satisfaction with the outcomes;
b) general: visual inspections of the main elements carried out on an annual basis that informs an organization’s budgets for maintenance programmes
and other maintenance;
c) detailed: a full inspection at intervals of not more than five years.

An inspection should be carried out by using a checklist made up of facility elements/sub-elements and arranged in way that supports safe working."

In essence this is usually translated to annual formal visual inspection & 5 yearly detailed structural survey.

Whilst the British Standard is not itself law, following it is a good way to demonstrate compliance with actual legislation, such as The Workplace (Health, Safety and Welfare) Regulations 1992, The Occupiers' Liability Act 1957 (as amended) and similar.