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Begin using it in some parts of testing of schema parse&compile.
(I don't think I'll try to push all usage of go-wish over to quicktest
in one flurry of diffs right now.  But it might be future work.)

The breaking point is that asserting on lists of errors using go-wish
was starting to exhibit Weird behaviors for only *some* error types.
It's likely that this would be addressable using some kind of go-cmp
customization, but go-wish doesn't really expose that ability.
The quicktest library does.

Switching to quicktest also brings a bunch of other nice features
along with it, like stack traces and other forms of additional info.
The holistic feel of it is also pretty similar to go-wish (particularly
since https://github.com/frankban/quicktest/pull/77 ), so many things
should be easy to switch.  I suspect I might want some more checker
functions to assert on types... but those should be easy to add as
a third party, either as a go-cmp.Transformer or a qt.Checker (and
then we can work on upstreaming at leisure, if appropriate).

In this commit, I'm handling the error list situation using a
go-cmp.Transformer to stringify all the errors.  This means that the
error *types* aren't checked, which is definitely loses ditches some
information... but the upside is that its easy to use the tests to
check that the eventual string of the error message is human-readable.
In this API, I think that readability is the higher priority.