Stabilize DDL contention sync tests without weakening coverage #24

Job	Run time
lint	44s
test	7m 41s
cross-compile (x86_64-apple-darwin, macos-latest)	1m 53s
cross-compile (aarch64-unknown-linux-gnu, ubuntu-latest)	1m 24s
cross-compile (aarch64-apple-darwin, macos-latest)	1m 41s
	13m 23s

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