v2.2: Fix flaky banking stage tests (backport of #5160)

[mergify]

Problem

Some banking stage tests try to process some transactions within the first block of a cluster with slow poh. If for some reason the transaction isn't included before poh ticks to the 2nd to last tick in the first block (takes about ~3.2s (8 * 400ms)), the test will never end because the poh loop stops recording transaction mixins.

Summary of Changes

• Make slow poh even slower by increase ticks per slot by a factor of 1024 (blocks will be over 6min long)
• Exit low power poh loop as soon as exit is set

Fixes #

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This is an automatic backport of pull request #5160 done by [Mergify](https://mergify.com).

[mergify]

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	both modified:   core/src/banking_stage.rs

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