tests: use common NodeContext in wallet_tests.cpp::ZapSelectTx

Summary:
D9989 broke src/test/test_bitcoin. This was not noticed until now
because most of the time we run the tests via ninja check, which
didn't fail (presumably because with ninja check there are not
concurrent wallets loaded at the same time).

This fixes it by using the common node context from BasicTestingSetup
to load the wallet, rather than creating a fresh NodeContext.

Test Plan:

ninja && ninja check
ninja test_bitcoin && src/test/test_bitcoin

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10029