Is it industry practice for CDN/ISP to do DDoS & Cyber drills or exercises

These provider need to let customer know the contact and how the severity of the event or attack can be escalated with a stipulated period based on the severity. The drill aspects is not a contractual default unless you asked for it and a formal exercise run through e.g. tabletop or execution based need to be deliberated early before awarding them. By default, these are not in the offering. Ask yourself that if the drill can be realised easily as the high Gbps is not easily generated.

Indeed exercise is required and there s value of it but you need to know the objective for the exercise - is it to exercise the incident handling protocol (exercise the CERT team),
- is it to verify the high Gbps handling capability (exercise the tiering to scale up as attack get worse),
- is it to validate the parties and information sharing timeliness (media and crisis comms involved) or
- is it to have all these mentioned run through for a maturity aspects (level up assessment wrt e.g. basic-standard, measured-up or  well-adapted benchmarks).  

It can be table top or actual simulation in a contained environment (snapshot) or an actual attack (which is unlikely because you be impacting the internet and other users causing false alarm to ISP and public). The exercise should not be only CDN but it need to be more holistic that can impact your online asset to cover L7 and L3/4 DDoS attack at minimal; and better still have use cases to drill further pertaining to supply chain compromised, malware infested CDN infrastructure, insider threat due to CDN provider/sub-contractor, unauthorised access and data leakage due to lapse of customer or provider oversight etc ...

Do check out the ENISA cyber exercise to get a better understanding what a "drills" should be
https://www.enisa.europa.eu/publications/exercise-survey2012/at_download/fullReport