Add support for multiple check commands

Currently, testservers_check_cmd_{baremetal,k8s} config key values are interpreted as single check commands.

While multiple commands could be provided by simply joining them with ';' that's non-ideal for a couple of reasons:

• The commands will run sequentially, adding to the total elapsed time of the deployment.
• The ';' character is "cursed" by the INI file format supported by configparser (i.e., comment prefix), and although we don't enable inline comments, it would still be rather confusing.

With this change, both config keys now interpret newlines in their values as specifying independent check commands. This works naturally with configparser's handling of multi-line values.

Why do this at all? This is to enable httpbb checks of the mwdebug-next deployment, to ensure coverage of PHP 8.1 at the testservers stage.

Points of note:

• The check name format now adopts a -{m}_of_{n} suffix to identify the individual check commands.
• httpbb check failures identify both the assertion that failed and the target host, such that it's still clear what failed.

Bug: T372604