test: Fix failing univalue float test

Summary:

Currently the test may fail for some compilers, because 1e-8 may not be possible to represent exactly/consistently.

Backport of core#29892.

Depends on D16296.

Test Plan:

ninja check check-univalue

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16297