Travis-CI, Cirrus-CI and some other CI vendors allow running integration tests on iOS simulators via a mac VM. For example see:
Of course, still can't debug. But debugging probably only necessary if developing iOS native plugins (which u probably aren't doing if not already on a mac). After all, one of the major promises of developing on Flutter is you can get many types of apps, developed on linux/windows, to run on android and iOS with the same code.
For example of deploying to iOS (and android) store via a CI vendor (or from command line if on a mac) see:
This uses the fledge command line interface: