Convert pre-existing stats_ext tests to new style
The regression tests added in commit
7300a69950 test cardinality
estimates using a function that extracts the interesting pieces
from the EXPLAIN output, instead of testing the whole plan. That
seems both easier to understand and less fragile, so this applies
the same approach to pre-existing tests of ndistinct coefficients
and functional dependencies.
Discussion: https://postgr.es/m/
dfdac334-9cf2-2597-fb27-
f0fb3753f435@2ndquadrant.com