Add test case for upgrading dcrlibwallet

[beansgum]

This should help in detecting compatibility errors and fixing bugs that may come up after upgrading to a higher version of dcrlibwallet.

[raedah]

There was specifically a problem with an upgrade that cause a db failure, a major failure that crippled use at start, but no test discovered this until it was manually found. A test that checks for basic initialization and basic operation could have found this.

[itswisdomagain]

Wallet db upgrades usually happen when a wallet is created or opened. As @raedah mentioned, I believe the tests we need are for basic initialization and operations, in this case - create, restore and open wallet tests.