tests: add (?) flag for optional lines
When the test engine fails to match output on a line marked with (?),
it will simply continue to the next expected line and try again. This
allows simplifying tests that have either version-specific or
non-fixed behavior, for instance:
$ coin-flip
heads (?)
tails (?)
(There's no form of back-tracking attempted, so optional matches
should be specific.)
changeset: {node}
user: {user}
date: {age}
summary: {desc}
{branches%branchname}{tags%tagname}{bookmarks%bookmarkname}
node: ({col}, {row}) (color {color})
{edges%graphedge}