[backport#19188] test: Avoid overwriting the NodeContext member of the testing setup [-Wshadow-field]

Summary:
NodeContext m_node is already a member of BasicTestingSetup which WalletTestingSetup inherits from via TestingSetup

Turns out Core had done this

Backport of Core PR19188

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, Fabien, deadalnix

Reviewed By: #bitcoin_abc, Fabien, deadalnix

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6835