fix broken backtracking behavior

don't even try recursing if the indentation isn't correct for it to
potentially be a subtest. this was causing exponential behavior since it
would try to parse a plan as a result, fail, then try to parse it as a
subtest result, and try to treat it as the start of a plan-first tap
stream when that shouldn't be possible at all because the indentation
should indicate that it's not a subtest