Tests: Fix test case name for URL-encoded character boundary in truncate_post_slug

[Soean]

Corrects the data provider label in Tests_Post_TruncatePostSlug: the test case with length=12 was incorrectly named "fifth" instead of "sixth", duplicating the key of the previous test case. As a result, the length=12 test was silently overwritten and never executed. This fix restores the correct label so both boundary cases are now covered.

Trac ticket: https://core.trac.wordpress.org/ticket/64894

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Props soean, mukesh27.

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[github-actions]

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[mukeshpanchal27]

Introduce in https://core.trac.wordpress.org/changeset/55208