Tests: Reset the `$wp_sitemap` global after each test, so that sitemaps are...

Tests: Reset the `$wp_sitemap` global after each test, so that sitemaps are re-initialized when the next test runs.

This ensures consistent results in query var tests, regardless of whether they are run in isolation or as part of a larger group of tests.

Props pbiron, peterwilsoncc.
Merges [48908] to the 5.5 branch.
Fixes #51154.
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git-svn-id: https://core.svn.wordpress.org/branches/5.5@48676 1a063a9b-81f0-0310-95a4-ce76da25c4cd