test: drop an unnecessary mkdir

Having a pre-existing directory confuse the clone abort logic and seems to not
be the intend of the test.

I improved the comment to highlight that.
15 jobs for topic/default/fix-test in 64 minutes and 18 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Tests
manual check-pytype-py3 #210869
allowed to fail manual
passed checks-py2 #210856

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passed checks-py3 #210857

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passed rust-cargo-test-py2 #210858

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passed rust-cargo-test-py3 #210859

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passed test-py2 #210860

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passed test-py2-chg #210867

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passed test-py2-pure #210862

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passed test-py2-rust #210864

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passed test-py3 #210861

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passed test-py3-chg #210868

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passed test-py3-pure #210863

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passed test-py3-rhg #210866

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passed test-py3-rust #210865

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  Phabricator
passed phabricator-refresh #210870

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