We largely use the corpus under test/corpus for
testing. Unfortunately, many of our tests have grown to depend on
having exactly this set of messages, making it hard to add new message
files for testing specific cases.
We do use a lot of add_message from within the tests, but it's not
possible to use that for adding broken messages, and adding several
messages at once can get unwieldy.
Move the basic corpus under tests/corpora/default, and make it
possible to add new, independent corpora along its side. This means
tons of renames with a few tweaks to add_email_corpus function in
test-lib.sh to let tests specify which corpus to use.
We will need this for improved folder search tests, but having some
folders should exercise our code paths better anyway.
Modify the relevant test accordingly to make it pass.
This reorganization triggers a bug in the test suite, namely that it
expects the output of --output=files to be in a certain order. So we
add the fix for that into the same commit.
This mainly involves sorting, although the case --duplicate=$n
requires more subtlety.
