test: Rename wallet.dat to wallet_test.dat
test: Rename wallet.dat to wallet_test.dat

Indicate that the file name is not hardcoded, and a little bit of safety
so that it never nukes the main wallet.

Suggestion by Marco Falke.

janko33bd/bitcoin,janko33bd/bitcoin,janko33bd/bitcoin,janko33bd/bitcoin,janko33bd/bitcoin,janko33bd/bitcoin