Remove test constructors or mark as TestOnly

[bentsherman]

Fell down a rabbit hole while working on #5045 . Trying to remove test constructors where it's easy to do so, otherwise mark them as TestOnly. Would be good to get rid of them over time as they suggest to me that a class isn't easily testable.

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[bentsherman]

@pditommaso do these changes look reasonable to you

[pditommaso]

@TestOnly is a nice way to mark those constructors, but why it requires changing so many tests? also seems there are some changes in charliecloud that looks unrelated.

[bentsherman]

I think I tried to remove test constructors in favor of using the real constructors as much as possible, where it didn't require too many changes to tests.

I think there was some issue recently where a test didn't catch a bug because it was using a test constructor and wasn't testing the behavior correctly. For this reason the test constructors seem like an anti-pattern, but in many cases they are convenient and it would be a hassle to remove them. So I just marked those with @testonly so that at least they are easier to track.

As for charliecloud, as far as I can tell that removed code is never used because registry is never set. I guess it's not related but I stumbled across it while making this PR.