[chronik] Fix -Wmissing-field-initializers in chronikbridge_tests.cpp

Summary: The test use field initializer to build a chronik_bridge::Tx but all the fields are not present causing this warning. This diff fills the blank to fix the warning, there is no change in behavior.

Test Plan:

ninja check

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18421