make gp tests almost equal

The CI has started failing ~10% of the time due to the recently added GP likelihood tests, e.g., here. This MR replaces the current test for equality with an approximate test, hopefully, this will lead to fewer random failures.

added Testing label

approved this merge request

approved this merge request

enabled an automatic merge when the pipeline for 6770c155 succeeds

merged

mentioned in commit 805eb0f1

mentioned in commit cjhaster/bilby@76a915ab

changed milestone to %1.2.0

mentioned in commit cjhaster/bilby@ffa8984c