Move TestDeleteFromMain from flat waiting times to active monitoring

[nagygergo]

Minor improvement, moves test case execution time from 150 to 70 seconds.

Also waitUntilMainBranchPackageRevisionExists was incorrectly implemented, it was not looking for a specific package having a main revision, but any package.

[nephio-prow]

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[nephio-prow]

Hi @nagygergo. Thanks for your PR.

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[nagygergo]

Closing in favor of #57 , which also offers a fix for the same thing.