Fixes Select Diff Seed bug that redirects to PSBTOverviewView

[newtonick]

Fixes Issue #449

Sign a PSBT that has already been signed with the same seed and you will see the PSBTSigningErrorView. However when you select "Select Diff Seed" you expect to be brought to PSBTSelectSeedView however PSBTSelectSeedView redirects to PSBTOverviewView because self.controller.psbt_seed has a value. This simple fix clears self.controller.psbt_seed prior to setting the next destination of PSBTSelectSeedView.

This also includes a unit test

[jdlcdl]

As of commit 288512a,

ACK tested.

I was able to reproduce this with SeedSigner's help, using sparrow. I had my ideas about this fix but I was advancing way too slowly thru PSBT flow tests, to prove that this bug exists before attempting a fix. Thank you Nick for great progress last night.

I'll note that this fix is just one commit ahead of important bug-fix merges made last night. I would encourage testing/ACKing and merge of this fix sooner than later so that we can keep up to date as we approach release.