We've made multiple attempts to stabilize the plans shown by commit
1bc0100d, with little success so far.  The reason for the remaining
instability seems to be that if a transaction (such as auto-analyze)
is running concurrently with the test, then get_actual_variable_range may
return a maximum value for "T 1"."C 1" that's far away from the actual max,
as a result of our having transiently inserted such a value earlier in
the test.  Because we use a non-MVCC snapshot to fetch the value (for
performance reasons), the presence of other transactions can cause that
function to return entries that are actually dead.

To fix, use a less extreme value in the earlier transient insertion, so
that whether it is visible or not won't affect the selectivity estimate.
The use of 9999 there seems to have been picked with the aid of a
dartboard anyway, rather than having a specific reason.

Discussion: https://postgr.es/m/16962.1523551784@sss.pgh.pa.us