qt/test: [FIX] Add forgotten Context setting in RPCNestedTests

Summary:
This is a partial backport of core#20323
https://github.com/bitcoin/bitcoin/pull/20323/commits/7e9e7fe56734d729ed7de39e880577b135dfd368

Test Plan: ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11170