REST API: Ensure rest_url() consistently has leading slash.

`rest_url()` inconsistent addes slashes to the passed path depending on whether the site has pretty permalinks enabled. Apart from being inconsistent, this also caused the unit tests to fail when pretty permalinks are enabled.

Props frank-klein.
Fixes #42452. See #41451.

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git-svn-id: https://core.svn.wordpress.org/trunk@42079 1a063a9b-81f0-0310-95a4-ce76da25c4cd