Port skipif.inc files to EXTENSIONS

Modernize IMAP tests

Use constants when appropriate, drop some ouputs which depends on env vars

Enable parallel testing.

Closes GH-6380

Fix #64076: imap_sort() does not return FALSE on failure

If unsupported `$search_criteria` are passed to `imap_sort()`, the
function returns an empty array, but there is also an error on

Fix #64076: imap_sort() does not return FALSE on failure

If unsupported `$search_criteria` are passed to `imap_sort()`, the
function returns an empty array, but there is also an error on the
libc-client error stack ("Unknown search criterion: UNSUPPORTED
(errflg=2)"). If, on the other hand, unsupported `$criteria` or
unsupported `$flags` are passed, the function returns `false`. We
solve this inconsistency by returning `false` for unsupported
`$search_criteria` as well.

Closes GH-6332.

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