prefork-interp: Move test script

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

diff --git a/perl/prefork-interp-test b/pm/Proc/Prefork/prefork-interp-test
similarity index 100%
rename from perl/prefork-interp-test
rename to pm/Proc/Prefork/prefork-interp-test