tests for choosing private key for signing, and pass phrase for signing

[tomholub]

I think the tests will be necessary. Let's do that later in a separate PR later. There are several tests that need to be written:

• ui test to check proper behavior when user inputs wrong pass phrase for message signing
• ui test to check proper behavior when user eventually inputs correct pass phrase
• ui test to check proper behavior when pass phrase already in memory
• unit tests to make sure that the right private key is chosen in various scenarios

Originally posted by @tomholub in #650 (comment)

[tomholub]

We already test most of this.