Add PHPUnit test for wp_ob_end_flush_all function

[pbearne]

This commit introduces a new PHPUnit test for the wp_ob_end_flush_all function. The function test is implemented in a new class, Tests_Functions_wpObEndFlushAll, and it will verify the correct behavior of wp_ob_end_flush_all by asserting the flushing of output buffering.

Trac ticket: https://core.trac.wordpress.org/ticket/60177

[github-actions]

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[mukeshpanchal27]

Thanks @pbearne for the PR. Left some feedback for consideration.