Tests: define UPLOADS constant for php unit.

[OlegApanovich]

Points the test suite at a dedicated wp-content/test-suite-uploads directory instead of the real wp-content/uploads, so test runs don't write to actual site content.

The directory wp-content/test-suite-uploads is deleted once before new tests run.

Adds an _upload_get_url() helper and updates tests that previously hardcoded wp-content/uploads.

Trac ticket: https://core.trac.wordpress.org/ticket/63532

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