Split tests into multiple files

[luisschwab]

@tvpeter in #214:

Considering how large the tests/wallet.rs file is, will it be ideal to someday organize the test cases into modules based on functionality? something like persistence, tx creation, etc

[tvpeter]

I will work on it

[luisschwab]

Just wait until the other gets merged so we don't go into a rebase loop.

[tvpeter]

Just wait until the other gets merged so we don't go into a rebase loop.

Sure.