CI: fix flakiness of some precision-based testing

[sadielbartholomew]

Some tests which pass locally fail on some (and possibly all in some cases) of the CI job runs corresponding to the full test suite running workflow. They therefore seem to be 'flaky' tests.

Most if not all of these tests involve precision equality testing, for example isclose with given atol and rtol and therefore floating point rounding or tolerance variations are likely the reason for the variation in result on different environments.

See also the following comments, for further context:

• the first bullet point of cf.aggregate "cells" keyword #674 (review);
• cf.aggregate "cells" keyword #674 (comment).