Tag change: Test suite: Introduce $tagpfx

Tag change: Test suite: Introduce $tagpfx

Currently this is `test-dummy' (the distro the tests work with).

Have checked that after this commit
   find tests -type f | xargs perl -i~ -pe 's/\$tagpfx/test-dummy/g'
puts everything back.  (This does not prove I have changed the right
set of occurrences, but it does suggest that each actual change is
good.)

No functional change.

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