[plt-scheme] Can't open old scm's on latest release

My joy at discovering that the performance problems that seemed to
plague DrScheme under Windows have now disappeared has been tempered
by the fact that I can't open my old solutions to HTDP.

I suspect it is something to do with the fact that they use the test
case facility.

Aside from reverting to an old release is there anything that can be
done.


Unknown snip class or version: "test-case-box%" version 2.

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