Hey Łukasz! :)
I am running arch-linux with cyrus-imapd as mail server for several years and now cyrus-imapd appeared in community-testing.
I am planning upgrade and switch from cyrus-imapd aur package to version from community-testing.
You can just continue using the AUR version until the package is actually moved to [community], so unless you want to help out testing the package there is no need to hurry switching to the one in [community-testing]. If however you want to help out by testing the new package you should read the wiki article[0] about the testing repositories, especially it is important that you also have to enable [testing] if you want to use [community-testing] because packages in [community-testing] link against libraries etc. from [testing] and you otherwise put your system in a state of being partially upgraded[1].
What is a recommended way to test only this package?
This is not something that is intended to work and is explicitly unsupported, as it has been pointed out in the other answer in this thread. The linked article[1] about partial upgrades provides some more insights on why that is.
If I enable community-testing in pacman.conf, then doing pacman -Su will still pull official packages from community until I explicitly request a version from testing?
The order in which repositories are placed in the pacman.conf is used to determine which version to take if there are two repositories offering the same package, see the following extract from pacman.conf(5):
The order of repositories in the configuration files matters; repositories listed first will take precedence over those listed later in the file when packages in two repositories have identical names, regardless of version number.
All the best, Chris [0] https://wiki.archlinux.org/title/Official_repositories#Testing_repositories [1] https://wiki.archlinux.org/title/System_maintenance#Partial_upgrades_are_uns...