Two bugs that were hiding each other, which is why both survived a month.

test_setlist.py ended in a module-level sys.exit(), pytest raises that during
collection, so 'pytest tools/tests/' aborted the whole directory and ran NOTHING
while 273 tests passed file-by-file. And what that invisibility concealed was
test_setlist's own failure, on a check whose name states the bug: OPAL's backlog
opens with '## SOUNDCHECK / -- Quand on decolle', entries() took every item under
the gig heading, and so every tool reading the setlist believed the set had 16
tracks opening on Quand on décolle. The recording has 15, opening on Ceci n'est
pas Une Bombe.

A test nobody can see failing is worse than a test that does not exist, because
its name is a correct bug report that everyone reads as reassurance.

Whole suite now runs in one command: 274 passed, 0 failed.