test dist/ modules against various older perls in CI

This currently has one major limitation: the perl-testers images
aren't threaded and don't have threaded alternatives, so the threads,
threads-shared and Thread-Queue dists are effectively untested on the
older perls.

The perl-testers images have a large number of test modules and their
dependencies installed, which means we're testing against the latest
versions of those dependencies (such as parent.pm) rather than against
the version of that dependency that shipped with that perl.

I tried to use the official perl images, but these only include a
small number of recent perls.  The git repository includes DockerFiles
for the older versions, but these aren't published to Docker Hub.

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