Tests: Merge [24251] from `_wp_filter_build_unique_id()` to...

Tests: Merge [24251] from `_wp_filter_build_unique_id()` to `_test_filter_build_unique_id()`, for consistency.

See #47407.
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git-svn-id: https://core.svn.wordpress.org/trunk@46606 1a063a9b-81f0-0310-95a4-ce76da25c4cd