[Cashtab] Patch flaky test in XECX redemption workflow

Summary:
Couple of things going on here.

1. the button is set to be disabled when agoraPartialTokenQty and agoraPartialMin are '0', but actually these values are empty strings on load (when the test was first written, they were '0').
2. Possible race condition depending on when state variables are set, perhaps the button is briefly enabled before it is disabled. Resolve with waitFor which is standard method of testing async behavior in react.

Test Plan: npm test; watch for CI failure to recur.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17772