[test] convert amount to XEC

Summary:
This amount was not converted from bitcoins to XEC in the initial backport (D10265)
This assertion ensures that the following calls to test_send will have enough funds.

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11920