Share fixtures across 'canonical' automated tests.

Sharing these fixtures results in a speed improvement of almost one minute per
run of the test suite.

My hope is that future WordPress developers will spend this extra minute with
their loved ones, for life on this earth is short, my friends, and the moments
you spend watching WP generate test data can never again be reclaimed from the
grizzled clutches of Time, and none of us are really getting younger, I mean,
geez, have you looked in the mirror lately, Gandalf?

See #30017.
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