Make it possible to run tests in parallel

salgado@delgadito~/.../monetas/btcwallet (vp) $ go test ./waddrmgr/
ok  	github.com/monetas/btcwallet/waddrmgr	2.713s

salgado@delgadito~/.../monetas/btcwallet (vp) $ go test -parallel 4 ./waddrmgr/
ok  	github.com/monetas/btcwallet/waddrmgr	1.629s