test: make it possible to have multiple corpora

test: make it possible to have multiple corpora

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.