tests: clarify test setup test in test-uncommit.t

Authored by martinvonz.

Description

tests: clarify test setup test in test-uncommit.t

I assume the "hg uncommit b" is there to prove that the working copy
is dirty before we try "hg uncommit --allow-dirty-working-copy b". It
seems clearer to put that check just before we run the actual test.

Differential Revision: https://phab.mercurial-scm.org/D6078

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