fix: in mbt, do not set outstandingDowntime for double-sign slashes (backport #1742)

[mergify]

Description

Closes: MBT test failure in https://github.com/cosmos/interchain-security/actions/runs/8472508023/job/23214757156?pr=1740

The issue occurred if there was first a double-sign slash request and then a downtime slash request for the same node.
In the model, I mistakenly always put outstandingDowntime for the node to true, so the downtime slash request was not sent.

This PR fixes this and adds a test for the issue.

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This is an automatic backport of pull request #1742 done by [Mergify](https://mergify.com).