Migrate SKIPIF -> EXTENSIONS (#7138)

This is an automated migration of most SKIPIF extension_loaded checks.

Modernize IMAP tests

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

Enable parallel testing.

Closes GH-6380

Fix #80213: imap_mail_compose() segfaults on certain $bodies

We have to cater to non-associative arrays where the key may be `NULL`;
we just skip these elements.

Closes GH-6315.