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This can make it a lot faster to run a single test.

You can run
    make CLEANUP=1 CLEAN_ONLY=YES
to clean everything without running any tests.

It was using ReadWriteMode, which made the driver think it was writing
to a file but not cleaning it. I don't think the fact it was ReadWrite
was important to the test, though.

It tried opening . for writing, which confused the driver into thinking
it was writing something but not removing it. So changed to use a
different directory instead.

Also stops it creating files it doesn't clean

Also look for files that are written but aren't cleaned

Removed some redundant imports and changed others from H98 to hierarchical modules.

Could have caused test failures if they both ran in parallel.

Just a small tidyup, and stops them both writing to prep.out.

The tests could have failed if they ran in parallel