Stabalize IMAP parallel testing, again

Drive by clean-up
Use less the default mailbox again

Fix parallel testing for IMAP

The script for the clean section was missing the $ Meta-character to only delete the default
mailbox when it's an exact match.

Reinstore parallel t

Fix parallel testing for IMAP

The script for the clean section was missing the $ Meta-character to only delete the default
mailbox when it's an exact match.

Reinstore parallel testing for more tests.

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Modernize IMAP tests

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

Enable parallel testing.

Closes GH-6380