Rename voteItemTestingContexts => voteItemProviders, because they provide vote items

Summary: And if they provide vote items, they should be called vote item providers

Test Plan:

ninja check-avalanche

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10309