tests: move verification closer to setup in test-copies-chain-merge.t
I found this test case really hard to read because it requires
scrolling back and forth between the setup and the verification. This
patch moves them closer together. There was also duplicated
descriptions for each test case (duplicated between setup and
verification). They had already become out of sync. I picked the
description from the verification, partly so the differences would be
highlighted in this patch.
Differential Revision: https://phab.mercurial-scm.org/D8377
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