Add tests for passing a UID instead of a message number

Backport IMAP test modernization to PHP-8.0 branch

Closes GH-6476

Fixed incorrect "skipif.inc" include

Fix Bug #80438: imap_msgno() incorrectly warns and return false on valid UIDs in PHP 8.0.0

Checking for a valid Unique ID (UID) cannot use the convenience macro as they might
be larger t

Fix Bug #80438: imap_msgno() incorrectly warns and return false on valid UIDs in PHP 8.0.0

Checking for a valid Unique ID (UID) cannot use the convenience macro as they might
be larger than the message number which has for maximum value the total number of
current messages available in the mailbox.

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