Move TestExt back into the main package

[nhz2]

This reverts #152 and is an alternative to #235

The current strategy of using an extension for testing utilities is causing some issues: Ref #223 #234

This PR moves the functions defined in the extension back into the main package, with slight modifications to avoid depending on Test or Random. The functions aren't documented, so I don't think slightly changing their behavior is breaking.

[mkitti]

Is this the right move? Moving this functionality into a subdir package sounds like a better idea.

[nhz2]

I copied the functionality into a subdir package in #235
It adds some extra complexity to the CI, and the subdir package needs to be registered before it can be used, but in the long run #235 is probably a more flexible solution.