refactor(tests): use absolute difference of dates to check if they are within a window

[lavafroth]

Changes

• Converts the resultant date to UTC.
• Takes the absolute difference of the result and the expected value.
• Checks if this value is less than (within) a window.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/stdbuf (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)

[sylvestre]

could you please explain why this change is needed? sorry it isn't clear to me :)

[lavafroth]

It's not a strict necessity. This way we need to define the threshold only once.

[sylvestre]

ok, code simplification then :)
thanks