[ci] de-flake app rewards test

[isegall-da]

Fixes https://github.com/DACH-NY/cn-test-failures/issues/6698

I actually disagree with @nicu-da .
The contract that fails this check is a leftover from a previous test, and AFAICT the only reason we're waiting for it to not exist is to not pollute this test. It eventually does get collected by a purchase-traffic automation, but that's not of interest to this test. I therefore propose to just not wait for that at all, and handle the existence of such contracts in the test.

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[nicu-da]

Makes sense, thanks!