tests: rename num to split-counter and move it to $TESTTMP in test-split.t

Let's not pollute working directory for no reason.
8 jobs for topic/default/test-cleanup in 16 minutes and 34 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed checks-py2 #245474

00:02:42

passed checks-py3 #245475

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passed doc #245480

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manual pytype #245473
allowed to fail manual
passed tests-py2-cext #245476

00:14:55

passed tests-py2-pure #245477

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passed tests-py3-cext #245478

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passed tests-py3-pure #245479

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