Update CmdCallbackTest to use the scaling factor (rebased onto develop)



This is the same as gh-1562 but rebased onto develop.

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This PR fixes the (hopefully) final occurrences of timeouts in the integration tests. It goes inline with the changes introduced in #1499. The best way to test this PR is to observe the state of the OmeroJava-integration-stable job after this PR gets into the build.

/cc @joshmoore