Fix two sources of flakiness in server_test
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Tests that exercise release-version file updates were previously flaky due to racing read-update (i.e., the watcher may observe file contents that is not valid JSON, and exit). This switches from simple write to staged-write/rename, which should be atomic in all contexts we care about.
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TestRunDelay used a maxDuration that did not take randomization into account. This bumps it to the theoretical max, plus a small buffer.