Post excerpt is causing unit test failures when searching for 0

[boonebgorges]

This is basically a duplicate of the problem solved in [36520] for 'post_content', except this time for 'post_excerpt'. See #35762, #31025.

[boonebgorges]

In 37280:

Tests: More explicit fixture content when testing s=0 query string.

After [36647], the unit test generator sequence can put a 0 into the
'post_excerpt' field of a post fixture, causing false positives.

See [36520] for a parallel fix involving 'post_content'.

Fixes #36622.