This PR adds unit test coverage for is_sitemap() from WordPress#12142, targeted at the 51543-is-sitemap-conditional branch so the tests show up directly in that PR.

Trac ticket: https://core.trac.wordpress.org/ticket/51543

What's added

tests/phpunit/tests/query/isSitemap.php — 12 tests (12 assertions, all green locally):

• WP_Query::$is_sitemap defaults to false
• The flag is set for the sitemap index route (sitemap=index) and for subtype routes (e.g. sitemap=posts&sitemap-subtype=post)
• WP_Query::is_sitemap() returns a bool
• An empty sitemap query var does not set the flag
• The sitemap stylesheet route (sitemap-stylesheet) is not flagged as a sitemap
• is_robots() takes precedence over is_sitemap() (matching the parse_query() elseif chain)
• A regular post query is never flagged as a sitemap
• A sitemap request is not treated as the home/front page (the practical motivation from #51542)
• The global is_sitemap() conditional tag reflects the main query (verified via go_to() + assertQueryTrue())
• is_sitemap() is false on the home page
• The global is_sitemap() triggers _doing_it_wrong() and returns false before the query is run

It also adds is_sitemap to WP_UnitTestCase::assertQueryTrue() next to the existing is_robots/is_favicon entries, for consistency.

Notes for reviewers

• The go_to()-based tests remove the render_sitemaps action from template_redirect so the renderer does not exit mid-test, and create published posts so the request isn't turned into a 404.
• That last point surfaces a behavioral edge case worth confirming: WP::handle_404() exempts admin/robots/favicon (class-wp.php), but not sitemaps. On a site with zero published posts, a sitemap request falls through to set_404(), which resets $is_sitemap via init_query_flags(). For normal sites the underlying query returns posts so the flag survives, but it may be worth deciding whether handle_404() should also exempt is_sitemap().

Test run

OK (12 tests, 20 assertions)

PHPCS (WordPress-Core) clean on both changed files; the existing Tests_Query_Conditionals suite still passes (121 tests) after the assertQueryTrue() addition.