metrics: tests: Fix flaky runtime_can_publish_metrics (#7279)

* metrics: tests: Fix flaky runtime_can_publish_metrics

When an re-org happens wait_for_blocks(2) would actually exit after the second
import of blocks 1, so the conditions for the metric to exist won't be met
hence the occasional test failure.

More details in:
  https://github.com/paritytech/polkadot/issues/7267

Signed-off-by: Alexandru Gheorghe <[email protected]>

* metrics: tests: Cleanup un-needed box pin

Signed-off-by: Alexandru Gheorghe <[email protected]>

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Signed-off-by: Alexandru Gheorghe <[email protected]>