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.)
edge: ({col}, {row}) -> ({nextcol}, {nextrow}) (color {color})