Avoid -Wshadow warnings in wallet_tests

Avoid -Wshadow warnings in wallet_tests

Warnings introduced by commit e2e2f4c "Return errors from importmulti if
complete rescans are not successful" and reported by Pavel Janík
<Pavel@Janik.cz> in https://github.com/bitcoin/bitcoin/pull/9773 and
https://github.com/bitcoin/bitcoin/pull/9827

wallet/test/wallet_tests.cpp: In member function ‘void wallet_tests::rescan::test_method()’:
wallet/test/wallet_tests.cpp:377:17: warning: declaration of ‘wallet’ shadows a global declaration [-Wshadow]
         CWallet wallet;