tests: better determinism in test-chg.t

chg tests fail pretty often with "Sample count: *" line disappearing.
It disappears because the sample count is zero, in which case a custom message is printed.
This commit makes the test succeed in that case.
17 jobs for topic/default/less-flaky-chg-tests in 54 minutes and 56 seconds (queued for 5 seconds)
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