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).
The error message was:
```
test_bitcoin: ../src/node/transaction.cpp:29: TransactionError BroadcastTransaction(NodeContext&, const Config&, CTransactionRef, std::string&, Amount, bool, bool): Assertion `node.connman' failed.
unknown location(0): fatal error: in "wallet_tests/ZapSelectTx": signal: SIGABRT (application abort requested)
../src/wallet/test/wallet_tests.cpp(869): last checkpoint: "ZapSelectTx" test entry
test_bitcoin: ../src/util/system.cpp:571: void ArgsManager::AddArg(const string&, const string&, unsigned int, const OptionsCategory&): Assertion `ret.second' failed.
unknown location(0): fatal error: in "walletdb_tests/write_erase_name": signal: SIGABRT (application abort requested)
../src/wallet/test/walletdb_tests.cpp(33): last checkpoint: "write_erase_name" fixture ctor
test_bitcoin: ../src/util/system.cpp:571: void ArgsManager::AddArg(const string&, const string&, unsigned int, const OptionsCategory&): Assertion `ret.second' failed.
unknown location(0): fatal error: in "walletdb_tests/write_erase_purpose": signal: SIGABRT (application abort requested)
../src/wallet/test/walletdb_tests.cpp(59): last checkpoint: "write_erase_purpose" fixture ctor
test_bitcoin: ../src/util/system.cpp:571: void ArgsManager::AddArg(const string&, const string&, unsigned int, const OptionsCategory&): Assertion `ret.second' failed.
unknown location(0): fatal error: in "walletdb_tests/write_erase_destdata": signal: SIGABRT (application abort requested)
../src/wallet/test/walletdb_tests.cpp(85): last checkpoint: "write_erase_destdata" fixture ctor
test_bitcoin: ../src/util/system.cpp:571: void ArgsManager::AddArg(const string&, const string&, unsigned int, const OptionsCategory&): Assertion `ret.second' failed.
unknown location(0): fatal error: in "walletdb_tests/no_dest_fails": signal: SIGABRT (application abort requested)
../src/wallet/test/walletdb_tests.cpp(122): last checkpoint: "no_dest_fails" fixture ctor
test_bitcoin: ../src/util/system.cpp:571: void ArgsManager::AddArg(const string&, const string&, unsigned int, const OptionsCategory&): Assertion `ret.second' failed.
unknown location(0): fatal error: in "wallet_crypto_tests/passphrase": signal: SIGABRT (application abort requested)
../src/wallet/test/wallet_crypto_tests.cpp(97): last checkpoint: "passphrase" fixture ctor
test_bitcoin: ../src/util/system.cpp:571: void ArgsManager::AddArg(const string&, const string&, unsigned int, const OptionsCategory&): Assertion `ret.second' failed.
unknown location(0): fatal error: in "wallet_crypto_tests/encrypt": signal: SIGABRT (application abort requested)
../src/wallet/test/wallet_crypto_tests.cpp(117): last checkpoint: "encrypt" fixture ctor
test_bitcoin: ../src/util/system.cpp:571: void ArgsManager::AddArg(const string&, const string&, unsigned int, const OptionsCategory&): Assertion `ret.second' failed.
unknown location(0): fatal error: in "wallet_crypto_tests/decrypt": signal: SIGABRT (application abort requested)
../src/wallet/test/wallet_crypto_tests.cpp(133): last checkpoint: "decrypt" fixture ctor
*** 8 failures are detected in the test module "Bitcoin ABC unit tests"
test_bitcoin: /usr/include/boost/thread/pthread/condition_variable_fwd.hpp:81: boost::condition_variable::~condition_variable(): Assertion `!posix::pthread_mutex_destroy(&internal_mutex)' failed.
Aborted (core dumped)
```
This fixes it by using the common node context from `BasicTestingSetup`
to load the wallet, rather than creating a fresh `NodeContext`.