Fix Bug #80800 imap_open() fails when the flags parameter includes CL_EXPUNGE

This also affected imap_reopen().
Add a supplementary test that the CL_EXPUNGE flag does have
the intend

Fix Bug #80800 imap_open() fails when the flags parameter includes CL_EXPUNGE

This also affected imap_reopen().
Add a supplementary test that the CL_EXPUNGE flag does have
the intended effect.

Closes GH-6732

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