test: fix key assignment in testing

[sainoe]

Description

During the testing setup, some consumer keys are assigned to the first consumer chain. This is achieved by calling the preProposalKeyAssignment function with the FirstConsumerId global variable. However, this variable is only initiated later in AddConsumer. As a result, the keys are mapped to an empty string "" as the consumerId, causing tests that rely on these consumer keys, e.g. TestRelayAndApplyDowntimePacket, to fail when run individually.

This PR refactors the testing setup to ensure preProposalKeyAssignment is called after FirstConsumerId is initialized.

Closes: #XXXX

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