fix(test): fix meta backup test

[zwang28]

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What's changed and what's your intention?

Previously this test relied on risectl's exit code. But after #15158 risectl always returns code 0 even on error.
This PR fixes the test by verifying ristctl's output.

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[zwang28]

@mikechesterwang @BugenZhao
RW Cloud also relies on risectl's exit code to decide whether op succeeds. Should we add the exit code back?

[BugenZhao]

But after #15158 risectl always returns code 0 even on error.

Oops. This is unexpected. I'll fix that.

[BugenZhao]

Hi. #15325 was merged. Do we still need this workaround?