disable only nonparallel seq scan.

Is there a way to force a meaningful parallel seq scan, or at least the
planning of one, when the planner wants a non-parallel one?

Usually I can do things like with with enable_* setting, but if I `set
enable_seqscan to off`, it penalizes the parallel seq scan 8 times harder
than it penalizes the non-parallel one, so the plan does not switch.

If I set `force_parallel_mode TO on` then I do get a parallel plan, but it
is a degenerate one which tells me nothing I want to know.

If I `set parallel_tuple_cost = 0` (or in some cases to a negative number),
I can force it switch, but that destroys the purpose, which is to see what
the "would have been" plan estimates are for the parallel seq scan under
the default setting of the cost parameters.

I can creep parallel_tuple_cost downward until it switches, and then try to
extrapolate back up, but this tedious and not very reliable.

Cheers,

Jeff